This code tries to prevent a large number of possible errors by
enforcing different restrictions on the messages that different
modules publish and subscribe to.
Some of these rules are probably too strict, and some too lax: we
should feel free to change them as needed as we move forward and
learn more.
This "publish/subscribe" layer sits on top of lib/dispatch, and
tries to provide more type-safety and cross-checking for the
lower-level layer.
Even with this commit, we're still not done: more checking will come
in the next commit, and a set of usability/typesafety macros will
come after.
