Tracing core

Sexp-all-the-things and tracing hooks.

In the end, I decided to follow Bob Harper and declare tracing a "benign effect". Since the stack itself cannot observe its tracing effects, there is no point in abstracting away the effect-type of tracing, so for the sake of sanity tracing is an actual, OCaml-level side-effect.

Currently the dumps look a bit verbose but it can be improved. Cstructs in particular should probably be done as hex.

Some sexp representations are unfinished.