Create a new watches type to keep track of the watches instead of
keeping two maps on the Watcher and accessing these directly.

This makes the bookkeeping a bit easier to follow, and we no longer need
to worry about locking map access as the watcher type takes care of that
now.

Came up in #472 where I want to keep track if a path was added
recursively, and this makes that a bit easier.

Also seems a bit faster:

	BenchmarkWatch-2          903709              7122 ns/op             194 B/op          3 allocs/op
	BenchmarkWatch-2          923980              6322 ns/op             196 B/op          3 allocs/op

Although that benchmark is very simply and only tests one code path;
just want to make sure it's not a horrible regression.