Use a workspace structure for reverie and the companion

[woodruffw]

The reverie repository itself currently has a single top-level Cargo.toml for the reverie library. The companion is tucked away in the companion/ subdirectory, with its own Cargo.toml.

We should restructure the repository to use cargo workspaces. That'll allow us to unify our dependencies and publish both reverie and the companion to crates.io.