Previously the arguments of the callback passed to `PortRPC.on` were
inferred as `any`. Make the callback type generic so that the caller is
forced to specify what types the arguments have.

Internally within PortRPC, use `unknown` rather than `any` for RPC
method arguments where possible.

Note that this change does not ensure that an RPC method is called (via `call`)
using arguments of the same types that the handler expects, but at least it is
now easier to check that the `call` and `on` uses match up and that arguments
are typed within the handler.