Further reduce interned type boilerplate

[detrumi]

Builds on top of #550, using a macro to generate the sequence structs, interned methods and Sequence impls.

[jackh726]

This is more of what I was expecting. But if we do this, it's easy enough to just generate the Sequence functions in the macro.

I guess this is a fundamental tradeoff: would we rather have more "code" or use traits to share "code". The trait route would result in less code generation, but requires we bring the trait into scope.

As a side point, I don't really like Sequence, I would instead prefer something like InternedSlice

[jackh726]

This LGTM. But I would probably like a review from @nathanwhit and/or @nikomatsakis, since they seemed like they liked the trait approach. But your discretion @detrumi

[nathanwhit]

I'm fine with this change 👍 . The main downside is that macros arguably make the code more opaque, but this is a pretty simple macro and definitely does have some ergonomic wins.