regex: avoid using literal optimizations when whitespace is detected

If a literal is entirely whitespace, then it's quite likely that it is
very common. So when that case occurs, just don't do (inner) literal
optimizations at all.

The regex engine may still make sub-optimal decisions here, but that's a
problem for another day.

Fixes #1087

crates/regex/src/lib.rs

@@ -5,6 +5,7 @@ An implementation of `grep-matcher`'s `Matcher` trait for Rust's regex engine.
 #![deny(missing_docs)]
 
 extern crate aho_corasick;
+extern crate bstr;
 extern crate grep_matcher;
 #[macro_use]
 extern crate log;