review HashSet

[infinity0]

containers' Set is defined as a separate data type where every field is strict so it is obvious there is no value to defining a "more strict" one here, since it would be exactly the same.

OTOH, unordered-containers' HashSet is defined as a newtype wrapper around HashMap _ (). Because HashMap internally uses a primitive Array# which is lazy in its contents, this means that HashSet may contain thunks even when the values are all (). This could be what's behind haskell-unordered-containers/unordered-containers#70.

If we instead define HashSet from our stricter HashMap, whose array operations all force the value, then we can avoid this potential problem.