eliminate PL_reglast(close)?paren, PL_regoffs

eliminate PL_reglast(close)?paren, PL_regoffs

eliminate the three vars
    PL_reglastcloseparen
    PL_reglastparen
    PL_regoffs
(which are actually aliases to PL_reg_state struct elements).

These three vars always point to the corresponding fields within the
currently executing regex; so just access those fields directly instead.

This makes switching between regexes with (??{}) simpler: just update
rex, and everything automatically references the new fields.