This PR performs some minor cleanups:

• _Regex_match1() took a flag _Full, but all call sites pass true, so we can just remove this argument. (I renamed the function even though it's not necessary, strictly speaking; there will be more changes to its signature.)
• _Matcher3::_Match took an argument _Full_match, but the value is the same for all calls on the same instance. Let's move this argument to the constructor and remove it from _Match2() (rename also not strictly necessary).
• Drive-by change: Add more const to updated function definitions.
• Drive-by change: The call sites affected by the other changes are updated to make _STD-qualified calls.