Reuse codepoint sets for identical character classes

These are very common and blowup memory usage at CTFE (that has no TLS
cache for that).
In any case it makes for slightly more compact memory footprint,
so a net gain regardless.
Merge pull request #2234 from DmitryOlshansky/regex-reuse-sets

std.regex - reuse codepoint sets for identical character classes