Handle fancy escapes in character classes

[robinst]

The code used to delegate parsing of escapes in character classes to the
regex crate. That meant that things that work outside of character
classes such as \< or \e would not work inside them.

To fix this, change it to parse the escapes ourselves and then map the
result.

[robinst]

Note that something like [\H] won't work even with this change, because it requires nested character classes, which are not yet supported in regex. The following pull request adds them, but it's not merged yet: rust-lang/regex#346