(REGEX_FREE_STACK, RESET_FAIL_STACK): Make them usable as an expression.

(REGEX_FREE_STACK, RESET_FAIL_STACK): Make them usable as an expression.
(enum re_opcode_t): Update description of succeed_n.
(PATFETCH): Always define.
(regex_compile): Use lookahead rather than PATUNFETCH (for repetition
operators, char classes, shy-groups and intervals).
Optimize special cases of intervals so as to only use succeed_n and
jump_n when really needed.
(re_compile_fastmap): Simplify handling of jump_n and succeed_n now
that we don't have to handle the special cases any more.
Simplify on_failure_jump handling as well.