feat!: support ignored path filtering

[shulaoda]

This is a reworked version of #114.

Why

A watcher cannot skip paths. Watching a project also watches node_modules and .git, which costs many watch resources and reports events nobody needs. chokidar solves this with ignored.

Changes

Adds Config::with_ignored(|path, kind| ...). An ignored path is treated as if it did not exist:

• It is never watched, scanned or reported. watch() and unwatch() of it return Ok(()) and do nothing.
• inotify, kqueue and poll do not descend into ignored directories. FSEvents and Windows watch recursively in the kernel, so they drop the events of ignored paths instead.
• The filter is asked once per path, never about its parents. To ignore a directory with everything inside it, it must return true for every path below it.
• kind is Dir, File, or Unknown when the watcher cannot tell (a missing path, every event on Windows).
• notify-debouncer-full gives the filter to its FileIdCache and does not add an ignored path as a root.
• FSEvents restarts its stream only when a watch was really added or removed, because a restart drops events.

Breaking change

Config (and debouncer-mini's Config) no longer derive Copy, PartialEq, Eq and Hash, because the filter is an Arc<dyn Fn>.

[h-a-n-a]

I just checked the test cases.

[sapphi-red]

Can we have the following tests for each backends?

1. trying to watch ignored directories
   • /foo/bar is ignored
   • /foo/bar/baz is added
   • expect that /foo/bar/baz is not watched
2. correctly ignore changes in ignored directory in watched directory
   • /foo/bar is added
   • /foo/bar/baz is ignored
   • expect that /foo/bar/baz/foobar is not watched