veyron2/ipc: 2 of n. 'Invoker and Signature' rationalisation.
veyron2/ipc: 2 of n. 'Invoker and Signature' rationalisation.

See https://veyron-review.googlesource.com/#/c/6254.
This adds Serve and ServeDispatcher, the only application
affected by this is wsprd.

Change-Id: Ic5dbcea9a0da5f840d61746955f3c553566e26f6

vanadium/go.v23,vanadium/go.v23,vanadium/go.v23