Language changes: #[unsafe_destructor] gets feature-gated, avoids some warnings.

* `#[unsafe_destructor]` now requires `#![feature(unsafe_destructor)]`. We can't easily remove it from `impl<'db> Drop for Cursor<'db>` (it uses an internal reference to satisfy the lifetime requirement), so we have to live with it for now.
* The internal reference in `Cursor` has been renamed to start with `_` and thus an unused field warning is suppressed.
* `#![phase(syntax, ...)]` became `#![phase(plugin, ...)]`.