Update rust

 - Update a renamed builtin. 
 - Enable nested block comments. Added test case for the latter as well.
A more future-proof way to express that would be:

    var BLOCK_COMMENT = hljs.inherit(hljs.C_BLOCK_COMMENT_MODE);
    BLOCK_COMMENT.contains.push('self');

`'self'` is a syntactic sugar for referencing itself.