Make `from/to_bitmask` not depend on `generic_const_exprs`

I.e. [these](https://github.com/rust-lang/portable-simd/blob/65cb2c90a0688c110d983a2dbb9932900cd6b5d9/crates/core_simd/src/masks/full_masks.rs#L115) are under `generic_const_exprs` which will take some time to stabilize.

Goal: find a way of re-writing those two functions without using `generic_const_exprs`