More Rust updates

Here are some more updates to the Rust ctags parser + filetype.

There was only one major issue fixed this time, which is that I completely forgot to deal with character literals (i.e. things like `'"'`). This caused Geany to occasionally see strings where there were none, amongst other issues. These things were somewhat rare in practice, so it took a lot of usage on my part to finally notice this bug.

One further minor change is that the hashbang syntax now has a special case now where if the first 3 characters are `#![` then it is not a comment, but rather some normal syntax. It still is highlighted as a comment out (since Scintilla does not have this fix yet, I am working on it) but it generates the tags correctly.

One new keyword this time.