OHWR.org general-cores WB crossbar requires table of address mask bits for each slave.
This MR adds support for generating such constants.
For now the expected use-case is that such address mask make only sense for direct top level slaves, there's no need to go down into hierarchy.
In particular, top level cheby definition may be just a collection of submaps like here toplevel.cheby
Mask constants are generated for all type files, but they may be useful only for VHDL constants package. To be discussed.