Removal of stdsimd

[tchataigner]

On February 5th, the feature stdsimd was removed from Rust in this PR: rust-lang/rust@ea37e80. It was split in multiple issues/features: rust-lang/stdarch#1486 .

This removal induces a compilation failure of plonky2_field in the latest stable and nightly releases of rust.

Ideally, we should adapt the codebase to this removal as some other have done (aHash for example) so that we can compile on stable channel. Another quick fix could involve setting a rust-toolchain file to some previous nightly releases.