New `ExecTracker` API

[apoelstra]

I propose that we extend and thoroughly document the ExecTracker trait. Currently it has several issues:

• It can be used to track left and right but cannot guide pruning (or directly be used to understand pruning)
• It can track jets but no other calls
• When tracking jets, it provides the input and output frames as &[UWORD] slices with no guidance on how to interpret them
• It can track dbg constructions (which are just asserts) where the CMR of the hidden node has some meaning...
• ...but only when the is_track_debug_enabled flag is provided, which is weird and unergonomic, but needed to avoid the overhead of frame->value conversions
• It has basically no documentation

Obviously we should improve the documentation. Beyond that, I think I can fix all the issues with the following design: #325