Second pass stabilization: vec

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

**Stable**:

* `dedup`
* `from_raw_parts`
* `insert`
* `into_iter`
* `is_empty`
* `remove`
* `reserve_exact`
* `reserve`
* `retain`
* `swap_remove`
* `truncate`

**Deprecated**:

* `from_fn`, `from_elem`, `grow_fn` and `grow`, all deprecated in
  favor of iterators. See rust-lang/rfcs#509

* `partition`, `partitioned`, deprecated in favor of a new, more
  general iterator consumer called `partition`.

* `unzip`, deprecated in favor of a new, more general iterator
  consumer called `unzip`.

A few remaining methods are left at experimental status.

[breaking-change]