Grammar: Fix loop expression labels

This fixes highlighting for lifetime labels in `loop` expressions. See [7.2.16](http://static.rust-lang.org/doc/master/rust.html#infinite-loops) in the Rust Reference Manual.
This is the example I included in `test.rs`:
```rust
'infinity: loop {
  do_serious_stuff();
  use_a_letter('Z');
  break 'infinity;
}
```
The previous grammar definition would erroneously match single-quoted strings in this context, which can cause entire blocks to be highlighted incorrectly.

Most of the problem was single-quoted strings: they do not exist in Rust's grammar. I changed the definition of `string.quoted.single.rust` to match [character literals](http://static.rust-lang.org/doc/master/rust.html#character-and-string-literals) instead, and changed `test.rs` accordingly.