Start restructuring to support generalized where clauses etc.
Thread a id to `Obligation`
Move the list of region obligations into the fulfillment context.
Introduce predicates but don't use them.
Write code for registering region obligations (but don't use it yet).
Switch to using predicates to drive checking. Correct various tests --
in most cases, just the error message changed, but in some cases we
are reporting new errors that OUGHT to have been reported before but
we're overlooked (mostly involving the `'static` bound on `Send`).
Remove diagnostic errors that were yielding warnings.
Fix the opt-out-copy behavior so that values with dtor etc are considered affine