Eliminate unsafeEqualityProof in CorePrep

The main idea here is to avoid treating
  * case e of {}
  * case unsafeEqualityProof of UnsafeRefl co -> blah
specially in CoreToStg.  Instead, nail them in CorePrep,
by converting

   case e of {}
     ==>   e |> unsafe-co

   case unsafeEqualityProof of UnsafeRefl cv -> blah
     ==> blah[unsafe-co/cv]

in GHC.Core.Prep.  Now expressions that we want to treat as trivial
really are trivial.  We can get rid of cpExprIsTrivial.

And we fix #19700.

A downside is that, at least under unsafeEqualityProof, we substitute
in types and coercions, which is more work. But a big advantage is
that it's all very simple and principled: CorePrep really gets rid of
the unsafeCoerce stuff, as it does empty case, runRW#, lazyId etc.

I've updated the overview in GHC.Core.Prep, and added
  Note [Unsafe coercions] in GHC.Core.Prep
  Note [Implementing unsafeCoerce] in base:Unsafe.Coerce

We get 3% fewer bytes allocated when compiling perf/compiler/T5631,
which uses a lot of unsafeCoerces.  (It's a happy-generated parser.)

Metric Decrease:
    T5631