Use DeBruijn indices for all type-level variables

[Nadrieril]

A few features involve nested binders:

• for<'a> clauses and function pointer types;
• GATs;
• dyn Trait, which I would like to encode as exists<T> T: Trait<Vars..>;
• closures, where I'd like to have one binder level per parent (to handle nested closures gracefully);
• trait methods, where I would like to explicitly separate the parameters from the parents from those of the method; this is ilkely necessary for provided methods.

Today we use binder groups + DeBruijn indices (just like rustc) for lifetime parameters. To support all these features cleanly I would like to do the same for type, const and trait clause parameters.