crc32: mark some unsafe functions properly, add some safety documentation

[inahga]

Some small nits I encountered in reviewing this code. Shoudn't be any actual functional changes.

• Any function/method that invokes an intrinsic on aarch64 or x86_64 is unsafe, because it has the precondition of ensuring the runtime processor supports the instructions, otherwise the result is UB.

  Results in a bit of an annoying proliferation of unsafe in pclmuqdg::Accumulator. There may be some fiddling that can be done to reduce the unsafe footprint, but it doesn't seem like a big deal for internal code.

• I set warn(unsafe_op_in_unsafe_fn), since otherwise unsafe operations can "hide" in unsafe blocks and thus not receive scrutiny.

• Added a couple of SAFETY comments. I eschewed safety comments for things that struck me as obvious, e.g. taking pointer to a slice that's just fed into an intrinsic.

(@folkertdev let me know if this kind of PR helps. This is all just techniques I was doing to review anyway, so figured they would be useful upstream too).

[folkertdev]

this is very helpful, thanks!