MutableCodeplug API

[masenf]

born out of #65 and #67

Looking forward to a new MutableCodeplug API, wrapping the codeplug-as-a-dict construct, while providing filtering and consistency methods.

currently all of the mutability of Codeplug exists in the filter function and many nested sub functions thereof. The logic is not easy to extend, overwrite, or control without significant code changes or extra parameters, etc.

If the CodeplugRecipe will expose the codeplug to callers at various points in the generation cycle, it needs to also expose a safe, extensible way to make modifications, that don't result in losing referential integrity across the codeplug objects.