Removed `Random a` constraint on `liftRand` and `liftRandT`.

The main reason, it seems, for users to need these lift functions is so
they can custom-generate from random-generators values when there is _no_ `Random`
instance. The previous constraint verily prevents that, though it does not really
need the constraint for its own implementation.
Eta-reduced `Rand` type synonym.

In the form `type Rand g a = ...` it is illegal to have partially applied `Rand g`,
but that is necessary to stack other transformers on top of `Rand`.
Merge pull request #7 from leftaroundabout/master

Removed `Random a` constraint on `liftRand` and `liftRandT`.