Stability: tweak rules for trait impls

There is currently no way to specify the stability level for a trait
impl produced by `deriving`. This patch is a stopgap solution that:

* Turns of stability inheritance for trait impls, and
* Uses the stability level of the *trait* if no level is directly
  specified.

That is, manual trait impls may still provide a directly stability
level, but `deriving` will use the level of the trait. While not a
perfect solution, it should be good enough for 1.0 API stabilization, as
we will like *remove* any unwanted impls outright.

r? @alexcrichton