This code fits better in check because it is checking that the impl
matches the interface. This allows us to avoid the awkward constructions
that lazilly collect traits and so forth. It also permits us to make
use of the results of variance inference.
It will be simpler to implement only one method for Ord, while we also
allow implementing all four Ord methods for semantics or performance
reasons.
We only supply three default methods (and not four), because don't have
any nice error reporting for the case where at least one method must be
implemented, but it's arbitrary which.
But note that default methods still don't work cross-crate (see #2794) --
this just makes it so that when a method is missing in a cross-crate impl,
the right error message gets printed.
Closes#3344
