This commit ensures that all backing memory allocated for the Guest CPU
is aligned to 256 bytes. This due to how gpu memory works and the heavy
constraints it has in the alignment of physical memory.

Instead of storing all block width, height and depths in their shifted
form:

block_width = 1U << block_shift;

Store them like they are provided by the emulated hardware (their
block_shift form). This way we can avoid doing the costly
Common::AlignUp operation to align texture sizes and drop CPU integer
divisions with bitwise logic (defined in Common::AlignBits).

Avoids potentially performing multiple reallocations (depending on the
size of the input data) by reserving the necessary amount of memory
ahead of time.

This is trivially doable, so there's no harm in it.

These can be generified together by using a concept type to designate
them. This also has the benefit of not making copies of potentially very
large arrays.

Allows for things such as:

auto rect = Common::Rectangle{0, 0, 0, 0};

as opposed to being required to explicitly write out the underlying
type, such as:

auto rect = Common::Rectangle<int>{0, 0, 0, 0};

The only requirement for the deduction is that all constructor arguments
be the same type.

While we're at it, also invert the conditional into a guard clause.

nullptr was being returned in the error case, which, at a glance may
seem perfectly OK... until you realize that std::string has the
invariant that it may not be constructed from a null pointer. This
means that if this error case was ever hit, then the application would
most likely crash from a thrown exception in std::string's constructor.

Instead, we can change the function to return an optional value,
indicating if a failure occurred.

These are already present within the header, so they don't need to be
repeated in the cpp file.

Makes the parameter ordering consistent, and also makes the filename
parameter a std::string. A std::string would be constructed anyways with
the previous code, as IOFile's only constructor with a filepath is one
taking a std::string.

We can also make WriteStringToFile's string parameter utilize a
std::string_view for the string, making use of our previous changes to
IOFile.

The file stream open functions have supported std::string overloads
since C++11, so we don't need to use c_str() here. Same behavior, less
code.

We don't need to force the usage of a std::string here, and can instead
use a std::string_view, which allows writing out other forms of strings
(e.g. C-style strings) without any unnecessary heap allocations.

Introduced in 72477731. This is only
necessary within header files.

Since the Mii database uses UUIDs very similar to the Accounts database, it makes no sense to not share code between them.

These two files were missing the #pragma once directive.

Uses arithmetic that can be identified more trivially by compilers for
optimizations. e.g. Rather than shifting the halves of the value and
then swapping and combining them, we can swap them in place.

e.g. for the original swap32 code on x86-64, clang 8.0 would generate:

    mov     ecx, edi
    rol     cx, 8
    shl     ecx, 16
    shr     edi, 16
    rol     di, 8
    movzx   eax, di
    or      eax, ecx
    ret

while GCC 8.3 would generate the ideal:

    mov     eax, edi
    bswap   eax
    ret

now both generate the same optimal output.

MSVC used to generate the following with the old code:

    mov     eax, ecx
    rol     cx, 8
    shr     eax, 16
    rol     ax, 8
    movzx   ecx, cx
    movzx   eax, ax
    shl     ecx, 16
    or      eax, ecx
    ret     0

Now MSVC also generates a similar, but equally optimal result as clang/GCC:

    bswap   ecx
    mov     eax, ecx
    ret     0

====

In the swap64 case, for the original code, clang 8.0 would generate:

    mov     eax, edi
    bswap   eax
    shl     rax, 32
    shr     rdi, 32
    bswap   edi
    or      rax, rdi
    ret

(almost there, but still missing the mark)

while, again, GCC 8.3 would generate the more ideal:

    mov     rax, rdi
    bswap   rax
    ret

now clang also generates the optimal sequence for this fallback as well.

This is a case where MSVC unfortunately falls short, despite the new
code, this one still generates a doozy of an output.

    mov     r8, rcx
    mov     r9, rcx
    mov     rax, 71776119061217280
    mov     rdx, r8
    and     r9, rax
    and     edx, 65280
    mov     rax, rcx
    shr     rax, 16
    or      r9, rax
    mov     rax, rcx
    shr     r9, 16
    mov     rcx, 280375465082880
    and     rax, rcx
    mov     rcx, 1095216660480
    or      r9, rax
    mov     rax, r8
    and     rax, rcx
    shr     r9, 16
    or      r9, rax
    mov     rcx, r8
    mov     rax, r8
    shr     r9, 8
    shl     rax, 16
    and     ecx, 16711680
    or      rdx, rax
    mov     eax, -16777216
    and     rax, r8
    shl     rdx, 16
    or      rdx, rcx
    shl     rdx, 16
    or      rax, rdx
    shl     rax, 8
    or      rax, r9
    ret     0

which is pretty unfortunate.

Allows the compiler to inform when the result of a swap function is
being ignored (which is 100% a bug in all usage scenarios). We also mark
them noexcept to allow other functions using them to be able to be
marked as noexcept and play nicely with things that potentially inspect
"nothrowability".

Including every OS' own built-in byte swapping functions is kind of
undesirable, since it adds yet another build path to ensure compilation
succeeds on.

Given we only support clang, GCC, and MSVC for the time being, we can
utilize their built-in functions directly instead of going through the
OS's API functions.

This shrinks the overall code down to just

if (msvc)
  use msvc's functions
else if (clang or gcc)
  use clang/gcc's builtins
else
  use the slow path

We don't plan to support host 32-bit ARM execution environments, so this
is essentially dead code.

The template type here is actually a forwarding reference, not an rvalue
reference in this case, so it's more appropriate to use std::forward to
preserve the value category of the type being moved.

Makes the return type consistently uniform (like the intrinsics we're
wrapping). This also conveniently silences a truncation warning within
the kernel multi_level_queue.

Introduced within 798d76f4, this only
really has an effect within header files.

Silences a -Wpragma-once-outside-header warning with clang.

Since C++17, the introduction of deduction guides for locking facilities
means that we no longer need to hardcode the mutex type into the locks
themselves, making it easier to switch mutex types, should it ever be
necessary in the future.

Many of these functions are carried over from Dolphin (where they aren't
used anymore). Given these have no use (and we really shouldn't be
screwing around with OS-specific thread scheduler handling from the
emulator, these can be removed.

The function for setting the thread name is left, however, since it can
have debugging utility usages.