Streaming regexes

[Ekleog]

As discussed on IRC :)

https://github.com/Ekleog/yuubind/blob/4f9a89808078f5d6a39adef86cf9066a8797d58d/smtp-message/src/lib.rs#L84-L129 is an example implementation of a combinator that applies a regex and returns Incomplete if the regex doesn't match yet but could match if more data were available.

Shall I try to make a PR out of it? :)