Generalize AST and ty::Generics to accept multiple lifetimes.
Introduce new variance inference pass that replaces (and generalizes) old
region-parameterization/variance inference. We now compute variance for
type parameters but do not make use of it (most of the way towards #3598).
Move comparing of impl methods against trait from collect to check.
This code fits better in check because it is checking that the impl
matches the interface. This allows us to avoid the awkward constructions
that lazilly collect traits and so forth. It also permits us to make
use of the results of variance inference.
Improve error reporting for region inference failures to make use of
all the information it has at its disposal. Unfortunately this also
reveals that we need to improve the reporting heuristics further,
as sometimes the errors it chooses to emit seem somewhat mystifying
and are not related to the actual problem.
Update various tests and libraries that were incorrectly
annotated.
Rename and modernize region enum names
Update FIXMEs with issue numbers