root testsuite: Regression test for problem exposed during most recent bootstrap.
root testsuite: fixed foo54_fine.rs to conform with rules of these tests.
lib-needsdrop: dynamic_lib needed drop of `result`.

(or maybe I was planning to give `Result` the `QuietEarlyDrop`
treatment?  Not sure; it can wait.)
rust-needsdrop: vtable: fixed bug I injected in an earlier broken refactoring.
rust-needsdrop: more refactoring to vtable API.