Refactor stdsimd

This commit:

* renames `coresimd` to `core_arch` and `stdsimd` to `std_detect`

* `std_detect` does no longer depend on `core_arch` - it is a freestanding
  `no_std` library that only depends on `core` - it is renamed to `std_detect`

* moves the top-level coresimd and stdsimd directories into the appropriate
  crates/... directories - this simplifies creating crate.io releases of these crates

* moves the top-level `coresimd` and `stdsimd` sub-directories into their
  corresponding crates in `crates/{core_arch, std_detect}`.