Second pass stabilization: slice

This commit takes a second pass through the `slice` module to
stabilize its API. The changes are as follows:

**Stable**:

* `as_mut_slice`
* `as_ptr`, `as_mut_ptr`
* `binary_search_by` (was: `binary_search`)
* `binary_search` (was: `binary_search_elem`)
* `chunks`, `chunks_mut`
* `contains`
* `ends_with`
* `first`, `first_mut` (was: `head`)
* `get_unchecked`, `get_unchecked_mut` (was: `unsafe_get`)
* `get`
* `is_empty`
* `iter`, `iter_mut`
* `len`
* `reverse`
* `sort_by`
* `sort`
* `split_at`, `split_at_mut`
* `split_mut`, `splitn_mut`, `rsplitn_mut`
* `split`, `splitn`, `rsplitn`
* `starts_with`
* `swap`
* `to_vec`
* `windows`

**Deprecated**:

* `head`, `head_mut` (renamed as above)
* `unsafe_get`, `unsafe_mut` (renamed as above)
* `binary_search_elem` (renamed as above)
* `partitioned`, deprecated in favor of a new, more
  general iterator consumer called `partition`.
* `BinarySearchResult`, deprecated in favor of `Result<uint, uint>`

[breaking-change]

Author: aturon