debugger serializes unused information

[thetarnav]

Currently, the walker in the debugger package is always serializing all the data.
Even stuff like signal values that are required only initially (later signals are updated separately) are mapped and included in the message sent to the extension.
Figuring out a way to cut unused information out of the walker could save some CPU time.
There are some tricky parts though:

• Logic for figuring out whether values should be included or not will have its cost too. The optimization may end up not helping at all.
• The extension isn't the only consumer of the debugger. Other utilities should also get a hold of the required data. What is unused for the extension, may be needed for other APIs.

Issue caused by the previous attempt at this: #3

[thetarnav]

Pretty much solved in #60 as the signals aren't a part of the mapped structure tree anymore.