Add `node_to_user_string`, `node_to_string` variant that drops id from output.
Added code to emit the gen/kill sets in source annotated form.

When you add `rustc_drop_obligations` as an annotation on a function,
it will print all of the drop-obligation manipulations as individual
compiler errors associated with the expressions that caused the effect
on the drop obligation state.
A pair of tests illustrating the `rustc_drop_obligations` instrumentation attribute.