Add script/CI to check code style

From the main commit:

Borrowed from Tracker, this script has different mode usages:
* --sha allows specifying a commit SHA, HEAD^ by default.
* --dry-run shows suggested changes in stdout, by default
  the local tree is changed.
* --rewrite amends the last commit, in addition to performing
  the changes.

This allows from simple style checks in the development tree,
automatically rewriting with the suggested changes (e.g. with
`git rebase --exec ./check-style -r`) and CI integration.

The script has per-function granularity, and works by re-styling
individually every commit, and inside every commit every chunk.