Fix typeck::infer::can_mk_assignty

[catamorphism]

A FIXME says: "FIXME---this will not unroll any entries we make in the
borrowings table. But this is OK for the moment because this
is only used in method lookup, and there must be exactly one
match or an error is reported. Still, it should be fixed."

[catamorphism]

FIXME is gone, assuming this is fixed.