This commit adds a `debug_scopes` container, which can collect metadata
about scopes in a design. Currently the only scope is that of a module.
A module scope can be represented either by a module and cell pair, or
a `$scopeinfo` cell in a flattened netlist. The metadata produced by
the C++ API is identical between these two cases, so flattening remains
transparent to a netlist with CXXRTL.
The existing `debug_items` method is deprecated. This isn't strictly
necessary, but the user experience is better if the path is provided
as e.g. `"top "` (as some VCD viewers make it awkward to select topmost
anonymous scope), and the upgrade flow encourages that, which should
reduce frustration later.
While the new `debug_items` method could still be broken in the future
as the C++ API permits, this seems unlikely since the debug information
can now capture all common netlist aspects and includes several
extension points (via `debug_item`, `debug_scope` types).
Also, naming of scope paths was normalized to `path` or `top_path`,
as applicable.
1a44645aef