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.
Improved syntax definition and snippets.

* Changes:
  Improved handling of attribute meta information.
  Add names to some of the place holders in snippets.
  Make Rust syntax definition closer to that of vim's
  Fix errors in Syntax Def's JSON-tmLanguage file.