cxxrtl: generate debug information for non-localized public wires.

cxxrtl: generate debug information for non-localized public wires.

Debug information describes values, wires, and memories with a simple
C-compatible layout. It can be emitted on demand into a map, which
has no runtime cost when it is unused, and allows late bound designs.

The `hdlname` attribute is used as the lookup key such that original
names, as emitted by the frontend, can be used for debugging and
introspection.