This change removes `bridgetree` from the workspace, so that it will no
longer be maintained in lockstep with changes to the
`incrementalmerkletree` crate. `bridgetree` will only receive bug-fix
changes in the future.
The `bridgetree-v0.2.0` tag was created with respect to an invalid
repository state, which did not actually contain the `bridgetree`
sources after they were factored out from the `incrementalmerkletree`
crate.
The structure of the `incrementalmerkletree` and `bridgetree` types has
not historically been stable, and as such it is not appropriate to
automatically derive serialization for these types.
The transitive dependency we have on `unarray` is forcing this update.
Also, update CI actions to eliminate dependencies on the unmaintained
`actions-rs` Rust CI tools.
Kris Nuttycombe
Jack Grigg