table, introduce a `FreeRegionMap` data structure. regionck computes the
`FreeRegionMap` for each fn and stores the result into the tcx so that
borrowck can use it (this could perhaps be refactored to have borrowck
recompute the map, but it's a bid tedious to recompute due to the
interaction of closures and free fns). The main reason to do this is
because of #22779 -- using a global table was incorrect because when
validating impl method signatures, we want to use the free region
relationships from the *trait*, not the impl.
Fixes#22779.
Refactor compare_impl_method into its own file. Modify the
code to stop comparing individual parameter bounds.
Instead we now use the predicates list attached to the trait
and implementation generics. This ensures consistency even
when bounds are declared in different places (i.e on
a parameter vs. in a where clause).
