Remove some outdated language features.

The following have been removed from Rust:

* The `~` and `@` sigils (use `box` and `box(Gc)` now).
* The `do` control keyword (use `spawn(proc() { ... })` instead now).
* The `priv` visibility keyword (things are private by default now).

Signed-off-by: Aaron Jacobs <atheriel@gmail.com>
Add `Drop` and `Freeze` as a built-in traits and `super` as a keyword.

Signed-off-by: Aaron Jacobs <atheriel@gmail.com>
Merge pull request #6 from atheriel/grammar_update

Remove some language features that have been taken out of the language in recent months (~, @, priv, do) and add a few things that might have been overlooked the first time (Drop, Freeze, and super).