Doc iter-tools/regex impl

[conartist6]

See: https://github.com/iter-tools/regex. It contains a proposed API as well as a polyfill implementation. Thanks to @jcbhmr for pointing this proposal out to me!

[conartist6]

To my knowledge backtracking is fully impossible to support in a proper streaming engine. Lookahead is tricky but possible to support. I also happen to support quantized repetition. Re2 simply blocks quantized repetition when streaming! I pay a price for that though, since I don't have any DFA optimization and the optimization that makes quantized repetition work OK blocks me from implementing a DFA optimization.

As a note if I do finish supporting lookahead all lookahead state would be fully preserved between subsequent matches.