Cleanup substitutions and treatment of generics around traits in a number of ways.

- In a TraitRef, use the self type consistently to refer to the Self type:
  - trait ref in `impl Trait<A,B,C> for S` has a self type of `S`.
  - trait ref in `A:Trait` has the self type `A`
  - trait ref associated with a trait decl has self type `Self`
  - trait ref associated with a supertype has self type `Self`
  - trait ref in an object type `@Trait` has no self type

- Rewrite `each_bound_traits_and_supertraits` to perform
  substitutions as it goes, and thus yield a series of trait refs
  that are always in the same 'namespace' as the type parameter
  bound given as input.  Before, we left this to the caller, but
  this doesn't work because the caller lacks adequare information
  to perform the type substitutions correctly.

- For provided methods, substitute the generics involved in the provided
  method correctly.

- Introduce TypeParameterDef, which tracks the bounds declared on a type
  parameter and brings them together with the def_id and (in the future)
  other information (maybe even the parameter's name!).

- Introduce Subst trait, which helps to cleanup a lot of the
  repetitive code involved with doing type substitution.

- Introduce Repr trait, which makes debug printouts far more convenient.

Fixes #4183.  Needed for #5656.

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