Note [Multiplicity annotations]

Multiplicity annotations are stored in the pat_mult field on PatBinds,
represented by the HsMultAnn data type

  HsNoMultAnn <=> no annotation in the source file
  HsPct1Ann   <=> the %1 annotation
  HsMultAnn   <=> the %t annotation, where `t` is some type

In case of HsNoMultAnn the typechecker infers a multiplicity.

We don't need to store a multiplicity on FunBinds:
- let %1 x = … is parsed as a PatBind. So we don't need an annotation before
  typechecking.
- the multiplicity that the typechecker infers is stored in the binder's Var for
  the desugarer to use. It's only relevant for strict FunBinds, see Wrinkle 1 in
  Note [Desugar Strict binds] in GHC.HsToCore.Binds as, in Core, let expressions
  don't have multiplicity annotations.