Tests first, on the parts branch

We build the same counting code as the main history of tally, in another order. Here each test comes with the code that it tests, and not after it. This history is on a branch of its own, parts, which leaves main where the project begins.

Each part ends with a section called What changed. It lists every file that the part changes, with a link to the file, and then the whole diff of each file, folded. A lockfile and a binary file have a link alone. The text above it explains the change.