Note [Respecting multiplicity when comparing types]

Generally speaking, we respect multiplicities (i.e. the linear part of the type
system) when comparing types.  Doing so is of course crucial during typechecking.

But for reasons described in Note [Linting linearity] in GHC.Core.Lint, it is hard
to ensure that Core is always type-correct when it comes to linearity.  So
* `eqTypeIgnoringMultiplicity` provides a way to compare types that /ignores/ multiplicities
* We use this multiplicity-blind comparison very occasionally, notably
    - in Core Lint: see Note [Linting linearity] in GHC.Core.Lint
    - in rule matching: see Note [Rewrite rules ignore multiplicities in FunTy]
      in GHC.Core.Unify