Refactor Unique to be represented by Word64

Fixes #22010 (closed)

I haven't touched FastMutInt and related uniques such as those used by FastString.

I'm using fromIntegral in a lot of places to convert Int to Word64. It might be good to use a function with a more specific type to avoid accidents. I'm also using fromIntegral to convert Word64 to Int in some places where I believe it is harmless.

There are three big todos left:

• genSym should generate Word64. This will probably require using unboxed primitives and probably even a new fetchAddWord64Addr# primitive. I just fallback to C on non-64-bit systems.
• The dominator Graph type needs to be changed from IntMap IntSet to Word64Map Word64Set.
• The EvalBreak constructor in the interpreter carries around and unboxed unique which needs to be boxed in withBreakAction.

Current problems:

• darwin builds still don't like how I define atomic_inc64

• tests are failing due to unknown known-key uniques.

• i386 build is failing with a weird panic #23537:

  iselExpr64(i386)
    _so3n::I64 - 16 :: W64

  (fixed with workaround; true fix would require new GHC release)

• JS build is failing with "ReferenceError: h$genSym is not defined" https://gitlab.haskell.org/ghc/ghc/-/jobs/1567116

• Release build is failing due to performance degradation on MultiLayerModulesTH_OneShot (+4.2%) and T13719 (+2.3%). !10568 (comment 505751)

Long-term todos:

• Make Dominators.hs prettier using type synonyms.
• See if some uses of Word64 should be replaced by Unique (and Word64Set by UniqueSet). (probably not worth it for now)
• Check if other uniques also need to change such as the uniques associated with FastStrings.
• Upstream Word64Map and Word64Set into containers.
• Re-evaluate changes in compiler/GHC/Types/Var.hs