Add ZSTD compression

[dbeinder]

Picking up on #16

The first two commits integrate zstd compression into NSIS

• Tested on MSVC 14 (Visual Studio 2022)
• In my testing, zstd at level 9-19 beats zlib & bzip2 in compression ratio and speed
• For the absolute best compression, lzma is still the best option, but everywhere else on the speed/size spectrum zstd seems to offer the best tradeoff with extremely wide tunability: https://facebook.github.io/zstd/
• Zstd stubs are 90kiB, this could be reduced to 75kiB with minimal compile options
  • Decompression speed is >200MB/s on my machine, but this could probably be improved
  • Zstd decompression is always single-threaded, unless exehead is extended to decompressing multiple files in parallel
  • Zstd requires between 2-10MB memory depending on zstd level (1-19)
• Compression in makensis is multithreaded
  • Compression speed at level 17 is 20MB/s on my machine
  • Without further changes, multithreading only helps for larger files or when using solid mode
  • Zstd compression uses 20MB per thread at level 9, and about 150MB per thread at level 19
• I have made some minimal changes to zstd and added a diff similar to 7zip
  • One change is to use fallback byteswap functions instead of calling into the CRT
  • All other changes are #ifndef EXEHEAD to strip unused features and reduce code size of the stubs
• The remaining uglyness is necessary to link zstd without a CRT into the stub
  • Zstd requires memcpy, memset, memmove, malloc, calloc, free, _allmul, and __allshl
  • For now, I have put simple implementations into exehead/crt_replacement.c
• Documentation updated, zstd License (BSD) added
  • Documented SetCompressionLevel script command to control compression level for zstd (and zlib)
• MakeNsisW & Zip2Exe updated

The newest 5 commits make everything Windows 95 compatible. Getting zstd to compile on VC6 would be a lot of work, and those changes would make updating zstd impractical - on the other hand, getting a EXE compiled by modern MSVC to run on Win95 is relatively easy:

• For the stubs, patch the PE header to allow running on Win9x and avoid calls to ConditionVariable kernel API
• For the Math plugin, instead of linking to a modern W95-incompatible CRT, embed libm and xprintf
• Moving to modern MSVC only increases the size of the existing stubs by 1-2kiB