Reorganise /core directory.

[AdamHillier]

What do these changes do?

This PR cleans up the directory structure, particularly /core.

The new directory structure is:

• core/
  • bconv2d/
    • optimized.h - what was previously /tflite/kernels/bconv2d_impl.h
    • reference.h - what was previously bconv2d_impl_ref.h
    • output_transform.h
    • padding_functor.h
  • bgemm/
    • bgemm.h - what was previously bgemm_impl.h
    • kernels.h - all the kernel registrations that were previously in bgemm_kernels.h and bgemm_kernels_arm.h
    • kernels_{arm32/aarch64}.h - the actual optimised bgemm code paths, unchanged from before
    • ruy_{pack/trmul_params}.h - unchanged
  • bitpacking/
    • Everything that was previously /core/bitpack_*
  • bmaxpool.h - unchanged
  • types.h - unchanged
• tflite/kernels/
  • Everything unchanged, except that bconv2d_impl.h has been moved to core/bconv2d/optimized.h

The changes I've made are what 'felt right' following a brief conversation with @Tombana about them but I happily welcome suggestions for alternative file names/placements!

How Has This Been Tested?

CI.

Benchmark Results

N/A.

Related issue number

N/A.