Add overloaded slice operations to the prelude

The new overloaded slice operations have replaced the various `slice`
methods on slice types. However, the relevant traits were not included
in the prelude, meaning that you would have to manually import
`std::ops::Slice` to get these operations -- an unintended regression.

This commit adds the traits to the prelude, as is done with most other
operator traits.

Closes https://github.com/rust-lang/rust/issues/17710