Reserved fields

As a user I want to specify multiple bit intervals in a register as 'reserved' to indicate that those intervals should not be used.

Under current rules, adding multiple fields with the same name will fail unless a field bit interval is also specified, so specifying the field bit interval to artificially associate multiple intervals with the 'reserved' field name could be a workaround.

Perhaps a reserved and/or explicitly unassigned field needs to be a special field instance that is allowed to have multiple instantiations in a register.