Merge branch 'feat-api-cmds-and-branching'

Implements the programmer-facing API to allow programmatic
specification of a synchronous round. The way in which these put/get
interactions are performed is in an initial shape. Perhaps this will
change in the future.

The second main set of changes is the addion of a 'fork' statement,
which allows explicit forking, and allowing multiple puts/gets over the
same transport link within a single sync round.