grep: add "perfect" smart case detection

This commit removes the previous smart case detection logic and replaces
it with detection based on the regex AST. This particular AST is a faithful
representation of the concrete syntax, which lets us be very precise in
how we handle it.

Closes #851

grep/src/lib.rs

@@ -19,6 +19,7 @@ pub use search::{Grep, GrepBuilder, Iter, Match};
 mod literals;
 mod nonl;
 mod search;
+mod smart_case;
 mod word_boundary;
 
 /// Result is a convenient type alias that fixes the type of the error to