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

tests/tests.rs

@@ -1687,16 +1687,6 @@ sherlock!(feature_419_zero_as_shortcut_for_null, "Sherlock", ".",
     assert_eq!(lines, "sherlock\x002\n");
 });
 
-// See: https://github.com/BurntSushi/ripgrep/issues/709
-clean!(suggest_fixed_strings_for_invalid_regex, "foo(", ".",
-|wd: WorkDir, mut cmd: Command| {
-    wd.assert_non_empty_stderr(&mut cmd);
-
-    let output = cmd.output().unwrap();
-    let err = String::from_utf8_lossy(&output.stderr);
-    assert_eq!(err.contains("--fixed-strings"), true);
-});
-
 #[test]
 fn compressed_gzip() {
     if !cmd_exists("gzip") {