dataflow: Now apply_gen_kill is pub, defend against degenerate case.
Add `opt_span_bug` to session, analogous to `opt_span_warn`.
Once you are beyond stage0, Gc<T> does not auto-coerce to &T.
rust-needsdrop: Various improvements to error reporting.

that is, error reporting on internal rustc issues in my hacked code.
Added another file to the root unit tests.