docs: link to rust items by name

[eric-seppanen]

• For items that are in scope, remove the explicit html path.
• For items not in scope, link to them by a rust path that does work (e.g. crate::Watches or libc::read).
• Fix incorrect reference WatchMask::IGNORED -> EventMask::IGNORED.
• Replace verbose [https://some-url/](https://some-url/) with the equivalent <https://some-url/>.

This fixes many broken links (some always broken, some only broken on docs.rs), and makes them more robust in the future (rustdoc emits warnings most items that don't resolve.)

Fixes #212.

[hannobraun]

It would be good to prevent this from happening again in the future. I've opened https://github.com/hannobraun/inotify-rs/issues/214.