Reduce memory when keeping track of debug frames

[pdawyndt]

Many debug frames contain the same objects. When these objects are large data structures, repeatedly storing these objects may take a lot of memory, which seems to drastically slow down the debugger. We might use memory in a more conserved way by storing references to objects in the debug frames, and making sure that objects that have not been modified are only stored once (similar to how GitHub stores object references). This may not only reduce the memory footprint, but also speed up the debugger.

Example of a slow debug session: https://dodona.be/en/courses/3355/series/36011/activities/413135075/ (try debugging a test for the function encode).