Just a trivial modernization that potentially avoids copying strings in certain scenarios.

Cleans up unused includes and trims off some dependencies on externals.

The follow-up to e2457418, which
replaces most of the includes in the core header with forward declarations.

This makes it so that if any of the headers the core header was
previously including change, then no one will need to rebuild the bulk
of the core, due to core.h being quite a prevalent inclusion.

This should make turnaround for changes much faster for developers.

General moving to keep kernel object types separate from the direct
kernel code. Also essentially a preliminary cleanup before eliminating
global kernel state in the kernel code.

Now that HandleTable doesn't directly depend on WaitObject anymore, this
can be separated from the main kernel.h header.