Use platform-specific methods for FileAccess reading and writing

[BlueCube3310]

Implements platform-specific variants of FileAccess's get_16, get_32, get_64, store_16, store_32, and store_64. Also adds MSVC-specific versions of BSWAPxx functions.

This results in a significant performance gain when reading or writing large amounts of binary data: ~5x faster writing and ~6x faster reading.

Benchmark results for writing data to a file:

Windows x64, build - production

data	master	PR
10000000 x uint64_t (78 mb)	1.70s	0.30s
50000000 x uint64_t (390 mb)	8.70s	1.80s
10000 x uint64_t (79 kb)	0.0020s	0.0007s
500000 x uint64_t big endian (4 mb)	0.085s	0.015s
500000 x uint64_t little endian (4 mb)	0.086s	0.015s

A small project for testing whether the changes work correctly: file_access_test.zip

Progress:

• FileAccessWindows
• FileAccessUnix
• FileAccessAndroid
• Minimal testing project

[m4gr3d]

The Android section looks good!

[Calinou]

Tested locally on Linux, it works as expected. Saved file checksums are identical to before with the test project.

[akien-mga]

Thanks!