Note [Fixed RuntimeRep]

Definitions:

  FRR.

    The type `ty :: ki` has a /syntactically fixed RuntimeRep/
    (we also say that `ty` is an `FRRType`)
      <=>
    the kind `ki` is concrete (in the sense of Note [Concrete types])
      <=>
    `typePrimRep ty` (= `kindPrimRep ki`) does not crash
    (assuming that typechecking succeeded, so that all metavariables
    in `ty` have been filled)

  Fixed RuntimeRep.

    The type `ty :: ki` has a /fixed RuntimeRep/
      <=>
    there exists an FRR type `ty'` with `ty ~# ty'`
      <=>
    there exists a concrete type `concrete_ki` such that
    `ki ~ concrete_ki`

These definitions are crafted to be useful to satisfy the invariants of
Core; see Note [Representation polymorphism invariants] in GHC.Core.

Notice that "fixed RuntimeRep" means (for now anyway) that
  * we know the runtime representation, and
  * we know the levity.

For example (ty :: TYPE (BoxedRep l)), where `l` is a levity variable
is /not/ "fixed RuntimeRep", even though it is always represented by
a heap pointer, because we don't know the levity.  In due course we
will want to make finer distinctions, as explained in the paper
Kinds are Calling Conventions [ICFP'20], but this suffices for now.