Design evaluation cache

[zimbatm]

A great idea by @Rizary.

Since prjfmt is responsible for traversing the filesystem, it can also keep track of the mtimes of each file during the classification. By doing so, it's possible to not re-format files that haven't changed.

The project needs to be changed to:

Load a cache file if it exists. The location should be $XDG_CACHE_DIR/prjfmt/eval-cache/<hex(hash(path))>.toml where path is the path to the prjfmt.toml file.

When traversing the filesystem, compare to the cache and remove all the files that haven't changed.

After formatting, check again the mtimes of the files that have been potentially touched. Update the cache with all the files.

[Rizary]

closed by #12