Nightly build fails due to removal of the unstable `stdsimd` feature

[Lachlan Deakin]

The latest nightly breaks this crate on this line because the unstable stdsimd feature has been removed:

crc32c/src/lib.rs

Line 23 in d70d4e2

 #![cfg_attr(nightly, feature(stdsimd))] 

The commit removing stdsimd is rust-lang/rust@ea37e80
Relevant tracking issues? rust-lang/rust#27731 and rust-lang/rust#48556