Remove the callback (cb) debug mechanism

This form made the dump more informative, but was awkward to use and
error-prone. Fortunately, we can get the same quality of information if we use
reflection and plain functions. The cb functions still remain to avoid risky
refactoring, but now just generate plain function definitions and invocations.
Add git/write-tree-entry function

Also simplify API of unit test utility method: cycle-bytes-hash
Update config database spec

Use more of the 'empty file' pattern rather than lists in plain files. Organize
text better to reduce redundancy. Move "let's try this later"-type ideas to a
separate section so they are not distracting.
Add parse-hash function for reading an ASCII SHA1 into the raw bytes