Allow a universe to bind many names

[leoyvens]

In rustc we allow an universal binder to bind more than one name, such as for<'a, 'b> fn(&'a u32, &'b u32) were both 'a and 'b have a DeBruijn index of 0. In chalk we also allow you to write that, but when moved into the environment 'a and 'b will have distinct universes (see fn instantiate_binders_universally and unify_forall_tys).

Currently chalk represents universal type in the environment directly by the universe index, so forall<T> { Goal(T) } becomes Goal(Ui) once the binder is into the environment. We might want to instead represent them with an index into a table in the environment, like we do for existential variables today, this would be one way to allow us to associate the universal type not only with a universe but with a particular name within that universe. Or maybe we could inline it by carrying two indexes Uij where j is the relative index of the name within the universe.

A motivation would be to u-canonicalize forall<T, U> { Goal(T, U) } and forall<T, U> { Goal(U, T) } into the same thing, as it should be easier to put the universals in a canonical order when you know they are at the universe.

Or at least that's how I understood it, ping @nikomatsakis