Improve Regex performance (mainly interpreted) (#449)

Improve Regex performance (mainly interpreted) (#449)

* Remove branches from tight inner interpreter loop in FindFirstChar

* Tweak RegexBoyerMoore.IsMatch

Reduce the checks needed and elimiate unnecessary layers of function calls.

* Remove IsSingleton optimization

This doesn't show up in real regexes and is just adding unnecessary complication to the code.  No one writes `[a-b]`... they just write `a`.  SingletonInverse is more useful, as you can search for any character except for a specific one, e.g. find the first character that's not a dash.

* Cache CharInClass results for ASCII lookups

* Improve codegen in a few places (and a little cleanup)

* Mark RegexInterpreter.SetOperator aggressive inlining

It's small but isn't getting inlined; it's only called in 4 places, but on hot paths, and inlininig it nets around an ~8% throughput win.