Instead of suppressing only trait errors that are "exact duplicates",
display only the "most high-level" error when there are multiple trait
errors with the same span that imply each-other.
e.g. when there are both `[closure]: Fn` and `[closure]: FnOnce`, omit
displaying the `[closure]: FnOnce` bound.
impls. This is a [breaking-change] (for gated code) in that when you
implement `Fn` (`FnMut`) you must also implement `FnOnce`. This commit
demonstrates how to fix it.
explicit form `Fn<A,B>` and now should use `Fn(A) -> B` or
`Fn<A,Output=B>`, but in some cases we get duplicate error
reports. This is mildly annoying and arises because of the main error
and another error from the projection. Might be worth squashing those,
but seems like a separate problem.
