RegExp rework - part 02

This is a third part of splitted PR #3223. (Other part are PR #3433, PR #3434)
This PR contains:
* general CRegExp optimizations, 
* addition of overloaded functions to deal with named submatches like with numbered, 
* some fixes, like checking submatches length and not trying to create string with negative length,
* RegFind improvement: now RegFind can be called with offset and max len limits,
* other enhancements.