Made signature of 'return' less polymorphic.

It is now the same as 'Prelude.$', which increases backwards-compatibility.
As this library doesn't compile with ghc-7.4, raised the 'base' dependency to '4.6'.
Got rid of 'PairObject k a b', which doesn't require a or b to be objects.
Added the obvious default implementations for 'first' and 'second', at last.