Remove SIMD functions

Benchmarking showed
that they were usually at least 2.5x slower on common platforms (x86_64),
and the maintenance overhead tracking nightly wasn't worth it.

src/lib.rs

@@ -1,5 +1,4 @@
 #![allow(clippy::uninlined_format_args)]
-#![cfg_attr(feature = "simd", feature(portable_simd))]
 
 mod error;
 mod standalone;