Move/rename repo

Given that this is shaping up to be a core shared library, it should live in the filecoin-project org:

- Either as a sub-crate in the builtin actors.
- Or as a new package? Maybe something like librfc?....
- Or something in the ref-fvm repo?

Right now, there's an awkward dependency detour where:

- The builtin actors depend on this repo.
- This repo depends on the FVM.

Most changes to the FVM's API will require a change change in _this_ repo before that change can be propagated to the builtin actors. This will be a non-issue once M2.2 (general programmability) ships as the FVM interfaces will have stabilized by then (or we'll have proper FVM "container" versioning, at the very least). But the current situation won't work for M2.1 and M2.2 because we're actively changing said interfaces.