feat: indicate functions which go on the bus - #2186

This puts in place an initial sharding implementation which is working,
except for the fact that sharding requires true checkpointing functions.
Furthermore, the test framework has not been updated yet for sharding.

This adds some checks to ensure that you cannot checkpoint on a
returning function.  Rather you have to checkpoint on a non-returning
function.

Signed-off-by: DavePearce <dave01001110@gmail.com>

This wires in the bus primitives for global functions, and updates the
reachablity check accordingly.  Tests for sharding are now operational.

Signed-off-by: DavePearce <dave01001110@gmail.com>