* std.crypto.hash.sha2: generalize sha512 truncation
Replace `Sha512224`, `Sha512256`, and `Sha512T224` with
`fn Sha512Truncated(digest_bits: comptime_int)`.
This required refactoring `Sha2x64(comptime params)` to
`Sha2x64(comptime iv: [8]u64, digest_bits: comptime_int)`
for user-specified `digest_bits`.
I left #19697 alone but added a compile-time check that digest_bits is
divisible by 8.
Remove docs which restate type name. Add module docs and reference where
IVs come from.
* std.crypto.sha2: make Sha512_224 and Sha512_256 pub
* make generic type implementation detail, add comments
* fix iv
* address @jedisct1 feedback
* fix typo
* renaming
* add truncation clarifying comment and Sha259T192 tests
8a36a1f913