Implement vec256/512

[TheNumbat]

Adds support for 256 and 512 bit vectors throughout the compiler. Generally has exactly the same semantics as 128 bit vectors, but larger. About half of the diff (+5k loc) is from enabling AVX/AVX2 in simdgen.

• 256-bit types are allowed with simd_beta. The basic tests for 128-bit have been duplicated for 256, but none of the operations in the other tests are implemented yet. AVX and AVX2 architecture extensions have been added and are enabled by default, since we already enable other Haswell extensions.
• 512-bit types are allowed with simd_alpha. They do not yet have tests and their usage will trigger a fatal error in emit. The AVX512F extension is also disabled by default.

Todo (before review):

• figure out how to make the runtime save ymm/zmm gc regs
• fix issues with external aligned stack args
  • when any parameter is 32/64 bytes, stack needs to be 32/64 aligned
  • the 32/64 byte stack parameters also need to be 32/64 aligned

For later PRs:

• align ocaml stack for vec slots
• static & reinterpret casts
• use vex encoding for sse when avx enabled (and evex if avx512)
• array accessors
• constants
• intrinsics
• avx512 mask registers
• avx512 tests