grep: upgrade to regex-syntax 0.5

This update brings with it many bug fixes:

  * Better error messages are printed overall. We also include
    explicit call out for unsupported features like backreferences
    and look-around.
  * Regexes like `\s*{` no longer emit incomprehensible errors.
  * Unicode escape sequences, such as `\u{..}` are now supported.

For the most part, this upgrade was done in a straight-forward way. We
resist the urge to refactor the `grep` crate, in anticipation of it
being rewritten anyway.

Note that we removed the `--fixed-strings` suggestion whenever a regex
syntax error occurs. In practice, I've found that it results in a lot of
false positives, and I believe that its use is not as paramount now that
regex parse errors are much more readable.

Closes #268, Closes #395, Closes #702, Closes #853

src/args.rs

@@ -9,7 +9,7 @@ use std::sync::atomic::{AtomicBool, Ordering};
 
 use clap;
 use encoding_rs::Encoding;
-use grep::{Grep, GrepBuilder, Error as GrepError};
+use grep::{Grep, GrepBuilder};
 use log;
 use num_cpus;
 use regex;
@@ -882,16 +882,7 @@ impl<'a> ArgMatches<'a> {
         if let Some(limit) = self.regex_size_limit()? {
             gb = gb.size_limit(limit);
         }
-        gb.build().map_err(|err| {
-            match err {
-                GrepError::Regex(err) => {
-                  let s = format!("{}\n(Hint: Try the --fixed-strings flag \
-                  to search for a literal string.)", err.to_string());
-                  From::from(s)
-                },
-                err => From::from(err)
-            }
-        })
+        Ok(gb.build()?)
     }
 
     /// Builds the set of glob overrides from the command line flags.