Add zero-arg flatMapIterable and concatMapIterable

[bemusementpark]

Implements #186

This introduces a convenient overload of flatMapIterable and concatMapIterable that allows the often-used flatMapIterable { it } to be abbreviated as flatMapIterable() and similarly for concatMapIterable.

This can save a number of classes and methods in an Android application.

[gildor]

I think it should be merged.
This idiom is pretty common in our code, but { it } looks really strange and confusing.

[gildor]

This can save a number of classes and methods in an Android application.

I agree with this proposal. but not sure that it can help to save classes/methods, actually methods count will be increased (+2 methods)

[bemusementpark]

Right. I figure those will be proguarded out if unused and if one of these are used more than once in a project, then we will be saving lambdas. The simplicity is more important.