Added pair combinator

[sayon]

Instead of using >>= as a generic but a bit cumbersome way to work with tuples of arbitrary length, you can use <~~> to pair up combinators.

[tekknolagi]

@pyrocat101 thoughts?

[Schmavery]

Missing the corresponding mli change:

val ( <~~> ) : ('token, 'a) parser -> ('token, 'b) parser -> ('token, ('a * 'b)) parser