rustdoc: generated trait impls for arrays are very noisy

[aturon]

For example, looking at the docs for Vec, some important trait implementations are hard to find in between all the PartialEq implementations for each size of fixed-size arrays: http://static.rust-lang.org/doc/master/std/vec/struct.Vec.html

Hopefully these impls will eventually become generic, but in the meantime we should consider some way of hiding or collapsing them in the docs.

[aturon]

cc @alexcrichton

[alexcrichton]

Dupe of #21660