Note [Desugaring coerce as cast]

We want the user to express a rule saying roughly “mapping a coercion over a
list can be replaced by a coercion”. But the cast operator of Core (▷) cannot
be written in Haskell. So we use `coerce` for that (#2110). The user writes
    map coerce = coerce
as a RULE, and this optimizes any kind of mapped' casts away, including `map
MkNewtype`.

For that we replace any forall'ed `c :: Coercible a b` value in a RULE by
corresponding `co :: a ~#R b` and wrap the LHS and the RHS in
`let c = MkCoercible co in ...`. This is later simplified to the desired form
by simpleOptExpr (for the LHS) resp. the simplifiers (for the RHS).
See also Note [Getting the map/coerce RULE to work] in GHC.Core.SimpleOpt.