Add type NonEmptySlice

[joseph-gio]

• Added the DST NonEmptySlice
  • Wrapper around a slice that guarantees it has at least one element.
  • Defines many helper methods that benefit from the non-empty restriction, such as first, split_last_mut, etc.
  • impl Deref<Target = [T]>, for the rest of the methods we expect.
• Change NonEmpty to impl Deref<Target = NonEmptySlice>.
  • This means the majority of NonEmpty's methods are now unnecessary, since we get them through the Deref impl.
• Added a significant amount of documentation.

[joseph-gio]

This may seem unnecessary, but I noticed the majority of the methods in this crate actually don't have anything to do with a non-empty vector, but a slice. So, I factored it out into a separate type which can hopefully be useful on its own.

[joseph-gio]

Okay, that's the last of the changes I want to make for this PR.

[yihuang]

LGTM, awesome.