Fix errors in Syntax Def's JSON-tmLanguage file.

Make Rust syntax definition closer to that of vim's

- Highlight var initialization in 'let' statements.
- Highlight extern crate names.
- Hightligth macros!

Add names to some of the place holders in snippets.

Improved handling of attribute meta information.

Minor clean-up

    - Fix highlighting fn brackets
    - Fix highligthing ':' in 'let foo: type = val;' initializations.