Note [Eta expansion of hasNoBinding things in CorePrep]

maybeSaturate deals with eta expanding to saturate things that can't deal
with unsaturated applications (identified by 'hasNoBinding', currently
foreign calls, unboxed tuple/sum constructors, and representation-polymorphic
primitives such as 'coerce' and 'unsafeCoerce#').

Historical Note: Note that eta expansion in CorePrep used to be very fragile
due to the "prediction" of CAFfyness that we used to make during tidying.  We
previously saturated primop applications here as well but due to this
fragility (see #16846) we now deal with this another way, as described in
Note [Primop wrappers] in GHC.Builtin.PrimOps.