fail!() used to require owned strings but can handle static strings
now. Also, it can pass its arguments to fmt!() on its own, no need for
the caller to call fmt!() itself.

errors

This commit does not remove `ty::arg`, although that should be
possible to do now.

Fixes #5656.
Fixes #5541.

- 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.

bare function store (which is not in fact a kind of value) but rather
ty::TraitRef.  Removes many uses of fail!() and other telltale signs of
type-semantic mismatch.

cc #4183 (not a fix, but related)

Move the replacement of bound regions out from `check_arguments` and into check_call / method_resolution

The typechecker previously passed around a boolean return flag to
indicate whether it saw something with type _|_ (that is, something
it knows at compile-time will definitely diverge) and also had some
manual checks for the `ty_err` pseudo-type that represents a previous
type error. This was because the typing rules implemented by the
typechecker didn't properly propagate _|_ and ty_err. I fixed it.

This also required changing expected error messages in a few tests,
as now we're printing out fewer derived errors -- in fact, at this
point we should print out no derived errors, so report any that
you see (ones that include "[type error]") as bugs.

and the fn_ctxt doesn't need any self_info field at all.

Step towards fixing `fn(&self)` (cc #4846) to have a distinct
lifetime.

(this will be needed for snapshotting at some point).

are currently inferred.  New rules are coming that will require
them to be explicit.  All add some explicit self declarations.