get rid of `Shrinker` proxy trait

[BurntSushi]

In a bygone era, I invented the Shrinker trait as a proxy so the shrink method of Arbitrary could return any iterator without regard to its underlying type. This was because Box<Iterator<A>> didn't itself implement Iterator<A>.

Now that DST is here and Rust has a more coherent story regarding trait object safety, it would be nice to remove this proxy trait and return Box<Iterator> instead.

I started working on this, but I ran into some bugs: rust-lang/rust#20605 and rust-lang/rust#20953. Once those are fixed, I think we can drop Shrinker.