capnproto-c++ has [`thisCap`](https://github.com/capnproto/capnproto/blob/44f5f5686962f3c8ff95618785d1998efd63a779/c%2B%2B/src/capnp/capability.h#L335). I imagine we could achieve something similar by adding a `get_self() -> Client` method to [`ParamsHook`](https://github.com/capnproto/capnproto-rust/blob/dd67e97405bcd23082f221f7a1a2d6b57de1e47d/capnp/src/private/capability.rs#L102) and [`Params`](https://github.com/capnproto/capnproto-rust/blob/dd67e97405bcd23082f221f7a1a2d6b57de1e47d/capnp/src/capability.rs#L155).