Added a shorthand for '(PairObject a b, Object{a,b,(a,b)})'.

Might make some signatures a bit less unsighty.
Made 'mapM_' / 'mapM' merely equivalents of 'traverse_?', confined to endofunctors.
Re-export 'mapM_' from monad module

(though it doesn't actually have much to do with monads).
Made 'Applicative' and 'Monad' instances a bit more straightforward to define.

'fpure' seems not needed at all, and since 'pure' is now equivalent to 'return' we might well at least default the latter to it.
Forgot to export 'fold'.