rustdoc: Self: Sized bounds in different crates are pruned

[alexcrichton]

For example Iterator::map should have a Self: Sized bound, but we're a little too aggressive about pruning out the Sized bounds.

For comparison core::iter::Iterator::map does indeed have the bound.

[apasel422]

This also applies to Sized bounds on traits.

[crumblingstatue]

This prompted a question on IRC about whether Clone requires Sized, because the Sized bound is not visible in the libstd documentation. It is visible in the libcore docs though.