NonEmpty vector implementation, with non-emptiness ensured by construction.

Inherits slices' methods through the Deref and DerefMut traits.

Vec's methods are manually overriden. Some important differences:

• len returns NonZeroUsize and is_empty always returns false.
• first(_mut), last(_mut), split_first(_mut), split_last(_mut) don't return Option.
• pop and remove return None if there is only one element.

For example usage, please look at the embedded unit testing.