The disciplines are, from the perspective of the ivl target, collected
into the design. Add functions for the target to scan the disciplines
in the design.
In the process, also clean up the handlng of design constants.
In the ivl_target API, the IVL_EX_BACCESS expression type gets some meat,
specifically references to the branch it accesses and the the nature to
be accessed on that branch.
This includes enough API to get the branch nexus bits and signals
and show them in the dump. This also includes creating the reference
ground for branch access functions that use the implicit ground.
NetTran devices must be collected into islands because they are all
a bi-directional mass. This is how vvp will process them and the code
generator will need a head start organizing them.
Stephen Williams