Keep slicing syntax compiling after nightly update

Some very minor fixes were necessary after updating to the latest rustc nightly.

* Enables the feature-gate for slicing
* Removes `use std::slice::Slice` since it is included in the prelude.

`cargo build && cargo test` work on `rustc 0.12.0-dev (683e40f35 2014-10-07 14:32:09 +0000)`