Max width of an Issie bus. There is no real need for any restriction,
since all code is bigint based, but this is a reasonable limit.
There are performance & UI issues for very large busses.
Here rather than in NumberHelpers, which is compiled after ComponentSlots: the bound a
width must satisfy is part of what a slot IS, and ComponentSlots.constraintsFor is where
that now lives. NumberHelpers.Constants.maxIssieBusWidth aliases this one.