Merge pull request #1471 from blackwhale/regex-cleanup-2

Regex code cleanup
do reverse of a bytecode in lookbehinds
backtracking matcher works with reversed bytecode
ditto with Thompson matcher
cleanup to keep lookahead/lookbehind more in sync for future
remove rarely useful debug hook
start on getting back-matcher into ctRegex, hit CTFE error
side-step Appender, 1 test failure to track
solve the usual direction puzzle of lookaround in lookbehind