This switches us over to using a fork of Olm latest.
The fork is mostly Anunay's work to add X3DH support, plus @jon's work to add Nix build support for macOS, plus some fixes from me.
To review this you'll probably want to check the fork's commit history on GitHub. We should probably set up some way to review the stuff in the fork, but I'm not sure what the best approach is yet. For now, I've left comments directly on @anunay's commit on GitHub.