Make builds incremental per action, by contentfeature/action-level-incremental

Every compile, archive, link and Zig build is now an action: a command
line with its directory and environment, the files it reads and the
files it writes. ActionCache keeps a record beside the first output of
each action, with the digest of the action and the SHA-256 of every
input and output. An action runs again only when:

- its command line, directory or environment changed,
- a file it read has other content: the declared inputs, and what the
  compiler included, from the depfile it wrote,
- or an output is gone or no longer what the action wrote.

Files are compared by content, not by time. Touching a file reruns
nothing; a translation unit recompiled to the same object (a comment in
a header, say) reruns neither the archive nor the program that use it.
Digests are kept per build by size and modification time, so each file
is read once.

- Zig writes no depfile, so every .zig file beside and below the root
  file, and the libraries it links, are the inputs of a Zig build. It
  no longer runs on every build.
- Staged headers and data files are copied only when their bytes
  differ.
- The .cmd records and the time comparisons of UpToDate are gone.

Tests: the action cache (command, environment, touched, changed and
restored inputs, altered outputs, forgotten records) and, with the C
compiler of the machine, a touch that runs nothing, a comment in a
header that recompiles two units but does not archive or link, and a
change of code that recompiles, archives and relinks.

0 files changed, 0 insertions, 0 deletions