spirv-std-const-generics crate to enable adt_const_params use

[Firestar99]

Counterproposal to @nazar-pc's PR #635

PR #635 is modifying spirv-std to use adt_const_params, which would break for anyone using a CPU-GPU shared crate that depends on spirv-std where the CPU target is compiled with stable. It would also be a significant breaking change, with any crate using those functions needing to be manually migrated.

This PR introduces the new crate spirv-std-const-generics, which reexports the entirety of spirv-std, but replaces individual functions with const-generic replacements. It should thus be equivalent to PR #635 but not break any existing setups. Instead, it's a drop-in replacement with a simple use spirv_std_const_generics as spirv_std in any crate. (Though any crate doing so will obviously no longer compile on stable.) You can also mix and match both spirv-std crates and it should work just fine.

Note on publishing: Our release-plz setup can only update existing crates, I'd need to claim the name and do some setup before we release. But I first want to know if we're claiming that name or if we want to rename the crate beforehand.

close #607
close #414

[nazar-pc]

That is a creative workaround, but I guess better than not supporting stable at all