- Move the 'current module' into the Node
- Split symbol group hierarchy into LocalsSymbolGroup
  tied to frame/thread and a separate, scopeless
  WatchesSymbolGroup
- Add infrastructure for removing symbols from a SymbolGroup,
  doing the index bookkeeping.
- Add method to synchronize watches to  WatchesSymbolGroup
  (iname/name map).
- Introduce watches commands for adding and dumping.
- Extend locals command to get watches as well.
- Add a dummy 'ErrorSymbolGroupNode' to use in case
  insertion fails.