rkyv does some optimization to not send all the data in
a string if you don't use it up.
xous assumes the same "pos" argument on the return type
thus a variable length string doesn't work as a return type
(you can send it, because you can specify a "pos" on the send
side).
fortunately, the application for the variable-length string
was getting predictions out of the predictor, and a short (31 element)
fixed-length array with a side companion of a len variable
should be more than sufficient for completion suggestions.
probably should be the *only* implementation because these are
only concerned with IPC calls that really require changes in
two separate crates, but I feel unconfident enough about the API
that for now I want the flexibility to experiment with non-default
APIs until I'm ready to freeze things.
