Use cached uniques for maps over TyCons and some other things.

The main idea is to avoid the indirection of Foo->Name->Unique and instead going Foo->Unique for certain lookups and membership tests.

There are still places where we can apply this pattern and some cleanup to do.

We introduce a few new map synonyms for this (TyConEnv/TyConSet) or use UniqFM/UniqueSet depending on context.