Our underlying implementation still looks pretty different, but
rather than implement some newly-needed APIs completely from scratch,
I copied in some of CPython's implementation.

The result is a bit messy (multiple ways of doing similar things),
but I think it's a step in the right direction.

Regardless, this commit adds "clean up thread-local storage when the
local object dies" functionality, as well as better cleanup when
there are multiple threads.  I think this should help with the fork
issues as well.