* The lib part should not deal with tools, or tool config, or running stuff in subprocesses. That stays in the CLI.
* Adds a fix_files function. This is the entry point.
* Adds a FileFixer trait with a single method to process a set of files. This is a high-level plugin point.
* Adds a ParallelFileFixer which implements the FileFixer trait and invokes a function to fix one file a time, in parallel.
* The CLI uses ParallelFileFixer.
The FileFixer trait allows environments (other than jj CLI) to plug in their own implementation; for example, a server could implement a FileFixer that sends RPCs to a "formatting service".
