Also drop the losing `instance MonadFail UniqSM`.

We redefine `getUniquesM` in terms of `Infinite` rather than `[]`, and define another method `getUniqueListM` for the use sites where we actually want a `[]`.
Thus, at many sites, we can avoid the partiality of the empty list case.

We also define `withUniques`, `withUniquesM`, and `withUniquesM'`, which traverse an arbitrary `Traversable` structure and introduce a `Unique` for each element.
This allows us to redefine various functions to operate on more appropriate types than `[]` and avoid further partiality (in the form of incomplete-uni-patterns).