A very simple crate, broken out solely for sake of rustc's requirement that
only export their procedural macros. (These function as something like compiler plug-ins.)
This crate exposes the internals that
spiceproc (the procedural-macros crate) depends on.
It is not designed to be particularly useful outside of Spice21.
pub annotations should be read as "public for Spice21".
SpProto Custom Message Trait