There are two scenarios where compiling AVX-512 C or assembly code might
not work:
1. There might not be a C compiler installed at all. Most commonly this
is either in cross-compiling situations, or with the Windows GNU
target.
2. The installed C compiler might not support e.g. -mavx512f, because
it's too old.
In both of these cases, print a relevant warning, and then automatically
fall back to using the pure Rust intrinsics build.
Note that this only affects x86 targets. Other targets always use pure
Rust, unless the "neon" feature is enabled.
The biggest change here is that assembly implementations are enabled by
default.
Added features:
- "pure" (Pure Rust, with no C or assembly implementations.)
Removed features:
- "c" (Now basically the default.)
Renamed features;
- "c_prefer_intrinsics" -> "prefer_intrinsics"
- "c_neon" -> "neon"
Unchanged:
- "rayon"
- "std" (Still the only feature on by default.)
This gives the assembly files the same prefix as the intrinsics files which
simplifies building when the build system should pick between the assembly and
the intrinsics files.
As part of this, get rid of the BLAKE3_FUZZ_ITERATIONS variable. I
wasn't using it anywhere, and it was leading to some compiler warnings
in --no-default-features mode.
