Monadic extension for flatMap

[Voltra]

flatMap is really handy, but the fact it has to return a Sequence make it a bit cumbersome to use. In this PR I introduce flatMapM which is a more "monadic" version of flatMap that allows the user to return any Iterable instead.

seq.flatMapM(f) is strictly equivalent to seq.map(f).flatMap(asSequence).