Fix bug in coherence that causes all cross-crate impls to be regarded as
inherent impls, not just those of the `impl Type` variety.
Make object types not implement associated trait.  Fixes #5087.
Remove trailing whitespace.
auto merge of #5252 : nikomatsakis/rust/issue-5087-make-trait-not-impl-self, r=pcwalton

Two changes:

- The first fixes an inconsistency in coherence whereby extension methods were added to the inherent methods table, but only in cross-crate scenarios.  This causes some minor fallout in tests and so forth.  In one case (comm) I added inherent and trait methods so as to avoid the need to import traits like `GenericPort` just to use a port.

- The second makes objects not implement the associated trait, as discussed in #5087.

r? @pcwalton