Eta-expansion can seriously blow up the size of the code, if there are
newtypes and coercions involved; see Trac #9020 for a particularly good
It may work out well in a full simplifier phase, when the additional
coercions can be optimized away, e.g. after inlining. But the gentle
phase does less, in particular no inlining, so chances are high that we
will blow up our code for on good reason.
If this change works out, there is no need for Trac #10319 anymore.