Improve some folds over Uniq[D]FM
* Replace some non-deterministic lazy folds with
strict folds.
* Replace some O(n log n) folds in deterministic order
with O(n) non-deterministic folds.
* Replace some folds with set-operations on the underlying
IntMaps.
This reduces max residency when compiling
`nofib/spectral/simple/Main.hs` with -O0 by about 1%.
Maximum residency when compiling Cabal also seems reduced on the
order of 3-9%.