regcomp.c: Use POSIXA, NPOSIXA
This commit optimizes character classes which are matched under /a or
/aa and consist of a single Posix class, into POSIXA or NPOSIXA regop
types. For example /[[:word:]]/a. Since [:ascii:] is always
ascii-restricted no matter what the charset modifier is, it is always
optimized.
These nodes should execute somewhat faster than a generic ANYOF node,
and are significantly smaller, taking 2 bytes instead of 12.
The flags field of the node structure is used to hold an enum indicating
which of the 15 Posix classes is being matched.