Replace pcre-light by regexp-tdfa.

The pcre-light package is a binding to a C library, so it is non-trivial to compile on Windows. I think this is the main reason for the "Unix systems only, currently." warning in the package description. In this pull request, I replaced pcre-light with regexp-tdfa (which is pure Haskell), and it compiles fine and seems to work.

Open Questions:

 * I ignore the `ungreedy` options. Are they important?
 * regexp-tdfa implements Posix regexps, but pcre-light implements Perl-style regexps. Is the difference important for ghc-core?
 * Is regexp-tdfa a good choice? It would be easy to use any other library that implements the common regexp API.