mgnp: fix outbound frame serialization interface

[hawkw]

from #2:

The interface exposed by outbound frames to the Wire is quite awkward, and needs to be reworked to make writing out serialized messages easier. This is a bit tough with the current tricky-pipe and postcard interfaces.

[hawkw]

i tried to fix this by using the erased-serde crate to add a serde::Serialize impl for tricky_pipe::SerRecvRef, but that crate's interface is way too constraining. we could possibly go down the path of essentially reimplementing our own version of erased-serde, but that sounds painful.

i think @jamesmunns might have some alternative ideas.