Replace per-save-point array snapshots with an undo trail

[natecook1000]

Creating a save point previously copied the four mutable register arrays (ints, positions, values, and storedCaptures) to enable restoration during backtracking. This led to a large amount of reference counting traffic and copy-on-write of the original register arrays whenever they were updated.

This change moves the restoration data from using full copies of each array in every save point, to tracking a log of the changes to each array in the processor itself and only storing the current state of the logs in each save point. This allows SavePoint to be a trivial type (almost) and removes all the copy-on-write traffic. Backtracking now uses the tracking logs to unwind the registers to the specific previous state.

This change also includes a minor refactor, moving the storedCaptures array to the Registers struct, alongside the other mutable registers. Future work includes breaking out the mutable registers into their own type, creating a flat structure for these instead of maintaining four different dynamically allocated arrays, and moving further toward making SavePoint trivial.