Improve cause information for NLL higher-ranked errors

This PR has several interconnected pieces:

1. In some of the NLL region error code, we now pass
   around an `ObligationCause`, instead of just a plain `Span`.
   This gets forwarded into `fulfill_cx.register_predicate_obligation`
   during error reporting.
2. The general InferCtxt error reporting code is extended to
   handle `ObligationCauseCode::BindingObligation`
3. A new enum variant `ConstraintCategory::Predicate` is added.
   We try to avoid using this as the 'best blame constraint' - instead,
   we use it to enhance the `ObligationCause` of the `BlameConstraint`
   that we do end up choosing.

As a result, several NLL error messages now contain the same
"the lifetime requirement is introduced here" message as non-NLL
errors.

Having an `ObligationCause` available will likely prove useful
for future improvements to NLL error messages.

compiler/rustc_middle/src/mir/query.rs

@@ -338,6 +338,11 @@ pub enum ConstraintCategory {
     OpaqueType,
     ClosureUpvar(hir::HirId),
 
+    /// A constraint from a user-written predicate
+    /// with the provided span, written on the item
+    /// with the given `DefId`
+    Predicate(DefId, Span),
+
     /// A "boring" constraint (caused by the given location) is one that
     /// the user probably doesn't want to see described in diagnostics,
     /// because it is kind of an artifact of the type system setup.