Resolve problems in finalization API

[brson]

The finalization API may need to change to support GC. I do not know the details. Nominating.

cc @pnkfelix
cc #11778

[pnkfelix]

P-backcompat-libs, 1.0, because it is strongly intertwined with #11778. (If that is revisited, this should be too.)

[brson]

Nominating for removal from 1.0. #11778 is not 1.0. Is there anything to do here?

[pnkfelix]

(closing, will move my issues to RFC repo issue.)