WISH: mold/trace

[hiiamboris]

To be considered a part of #134

I have a few needs so far:

1. Native mold freezes the Spaces program with its huge useless output (all the time when working with Spaces)
2. To provide specialized output for Space objects and some special values (when debugging to keep the output neat)
3. To know in the resulting text where each original value starts and ends (in ParSEE to color any-block input regions inside displayed text)

[hiiamboris]

Absence of this feature becomes a blocker for further ParSEE evolution (another one is red/red#4524 (comment)).

To support displaying list input modifications ParSEE needs to map incremental changes of input to incremental changes of this input's molded text (what the user sees).

For top-level input modifications each modification requires me to make two keyframes: before and after a modification. Each keyframe holding a copy of the molded input.

For deeper modifications I currently see no way of achieving this mapping. Using mold/trace I could have made a map where all input positions are located in the molded text, even for the deeper lists, and could then trivially reflect their removal into the molded text. Reflection of insertions would have been a bit more involved, but doable as long as mold of a single value is equivalent to its mold inside the container.

For now, I'll try to make my own mold-trace, which should be feasible since in ParSEE I don't need all the features of mold.