Remove `impl &Type`

[bstrie]

<@nmatsakis> I'm not sure whether impl &Type should even be allowed, but maybe
             there's a good reason for it.
<@pcwalton> bstrie: impl &Type is going to be removed
<@pcwalton> well, I would like it to be
<@pcwalton> it's on my list of things to forbid
<@pcwalton> the concept of "base types" is a thing imported from Go that makes
            no sense anymore
<@pcwalton> with explicit self

[catamorphism]

@pcwalton fixed this -- closing