Generalizing taping infrastructure (e.g. tools for recording execution traces)

From the Discourse announcement I made a while back:

Internally, ReverseDiff contains facilities for recording execution traces of native Julia code to reusable, compilable instruction tapes, as well as mechanisms for propagating values "forwards" and "backwards" through these tapes. Since these tapes can be analyzed as computation graphs, my hope is that this infrastructure can eventually be rendered useful for non-AD purposes, such as performance optimization, scheduled parallel execution, and constraint programming.

I've opened this issue to track this effort. Right now, TrackedArrays/TrackedReals and the instruction tape are written specifically for differentiation purposes. It will require a lot of thought and experimentation to figure out the right way to restructure this code to be generally useful for non-AD purposes. Eventually, this code might be factored out of ReverseDiff and into a different package.

cc @dpsanders (who might someday find this stuff useful for interval constraint programming).

Start by examining the TrackedArray and TrackedReal implementations and the instruction tape described in this issue, then read the linked Discourse announcement for context. The work is exploratory: determine how to restructure the differentiation-specific infrastructure for broader uses such as execution-trace analysis, and whether it should eventually move into another package.