optimisations to newUnique

It turned out that newUnique was wasting one node of the splittable
uniq supply per invocation: it took the current supply, split it, used
the unique from one half and stored the other half in the monad.  In
other words, the unique in the supply stored in the monad was never
used.

This optimisation fixes that and adds a bit of strictness, which
together lead to small reduction in allocations by the compiler, and
possibly an improvement in residency (hard to tell for sure when GCs
move around).