rt: add a gc field to task
rustc: remove old bitrotted gc system.
core: fix bug in reflect visiting vec rather than unboxed vec.
core: add hex-writing helper to WriterUtil
core / rt: reserve / realloc managed-unique pointers through at_vec.
core: rewrite cleanup to contain a conservative GC (not active)
core: hook up local heap malloc/free/realloc to notify the conservative GC.
rustc: stop emitting refcount code, fail if anyone tries to explicitly free @ boxes.
rustc: add zero_root upcall, use to keep roots alive long enough (and zero after)
rustc: clean up unused imports
DO NOT LAND: gc-diagnostics to set interned tokens as gc-precious
DO NOT LAND: gc-diagnostics to keep pp interner alive a bit longer.
DO NOT LAND: gc-diagnostics to do more rooting than normal