[audio_core] Update for firmware 15.0.0

common/fixed_point: Minor interface improvements

savedata_factory: Detect future save data paths

kernel: fix slab heap ABA

Kernel Multiprocess (Part 1) - Persist memory & core timing

video_core: implement 1D copies based on VMM 'kind'

Ensures that a fixed-point value is always initialized

This likely also fixes several cases of uninitialized values being
operated on, since we have multiple areas in the codebase where the
default constructor is being used like:

Common::FixedPoint<50, 14> current_sample{};

and is then followed up with an arithmetic operation like += or
something else, which operates directly on FixedPoint's internal data
member, which would previously be uninitialized.

Given these are just moving a raw value around, these can sensibly be
made constexpr to make the interface more useful.

These shouldn't throw and can influence how some standard algorithms
will work.

Marks member functions as discard, where ignoring the return value would
be indicative of a bug or dead code.

This calls round_up(), which is a non-const member function, so if a
fixed-point instantiation ever calls to_uint(), it'll result in a
compiler error.

This allows the member function to work.

While we're at it, we can actually mark to_long_floor() as const, since
it's not modifying any member state.

Collapses all of the comparison functions down to a single line.

Makes a few things a little less noisy and removes the need for SFINAE
in quite a few functions.

CMake: Try add_library "lz4" if "lz4::lz4" is unavailable