RegExp handling of internal vs. external string representations

[natevw]

At the time of #542, RegExp did not really implement any index-related fields (e.g. lastIndex) so its proper handling has not been audited. Also need to take a look at how it handles the string encoding situation in general.

Bonus: also should look into supporting ES6 'u' flag and friends!

[tcr]

First step is removing the mysterious mbtowcs function that we engrain in runtime. @natevw want to review #566 ?

The next step in this PR is to replace the wchar.h reliance by hsregex—I believe it only uses toupper and tolower functions. No idea what wchar's toupper equivalent is or whether it even implements Unicode.