Constrained interfaces require little change to core carp. In rare cases
(illustrated in test/interfaces.carp) we need to make our functions
slightly more specific. This weakens inference slightly, but in my
opinion is a reasonable case of weakness and not a fault of the type
system.
Exceptional cases that were typed `a-> a` that is, cases that relied on
the previous typing errors related to interfaces, as might be expected,
no longer work (as is the case for the higher-order use of fmap in
examples/functor.carp).
Aside from that, nothing changes.
