regex: small cleanups

Just some small polishing. We also get rid of thread_local in favor of
using regex-automata, mostly just in the name of reducing dependencies.
(We should eventually be able to drop thread_local completely.)

crates/regex/src/non_matching.rs

@@ -7,7 +7,7 @@ use {
 };
 
 /// Return a confirmed set of non-matching bytes from the given expression.
-pub fn non_matching_bytes(expr: &Hir) -> ByteSet {
+pub(crate) fn non_matching_bytes(expr: &Hir) -> ByteSet {
     let mut set = ByteSet::full();
     remove_matching_bytes(expr, &mut set);
     set