Reduce boilerplate for interned type definitions

[nathanwhit]

Right now we repeat code like this in at least 4-5 places across chalk-ir. This not only creates bloat, making the definitions inchalk-ir harder to read, but also makes adding new interned types very tedious. We should find a way to reduce the amount of code duplication and boilerplate here.

[nikomatsakis]

Macros or derive might be an option here -- perhaps a trait for interned sequences? Something like that.

[nathanwhit]

@rustbot claim

[nathanwhit]

Closed by #554.