Implement some garbage collector

[brson]

I don't see any issue about just getting garbage collection working, which it increasingly looks like we want to have done for 1.0. Nominating.

[flaper87]

related to #11399

[huonw]

The near-identical #2997 was accepted for P-low.

[pnkfelix]

Accepted for 1.0, assigning P-backcompat-libs (but we may revisit priority assignment later...)

[brson]

Nominating for removal from milestone. Does not seem possible.

[pnkfelix]

We should create a separate ticket for ensuring that our allocator design, whenever it is created, properly supports a reasonable form of garbage collection.

Taking off the 1.0 milestone and removing the P-backcompat-lib; reassigning P-high.