#556 Rust edit mode fixes

[Jordan Miner]

Here are two fixes to the Rust edit mode:

• I forgot that block comments in Rust can be nested.
• Between the beta and the final version, the is and us integer suffixes were changed to isize and usize.

Here's the updated example file:

https://gist.github.com/jminer/401fafd13107d512a40f

(It seems the Github syntax highlighting doesn't support the nested comments.)

[Alan Ezust]

Committed #23918