* Attempt to fix tests on master
* Make all doctests use items from the real `std` rather than this
crate, it's just easier
* Handle debuginfo weirdness by flagging functions as `no_mangle` that
we're looking for instructions within.
* Handle double undescores in symbol names
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}`.
