Clean up `libcollections::VecMap`

- Introduce a named type for the return type of `VecMap::move_iter`
- Rename all type parameters to `V` for "Value".
- Remove unnecessary call to an `Option::unwrap`, use pattern matching instead.
- Remove incorrect `Hash` implementation which took the `VecMap`'s capacity
  into account.

This is a [breaking-change], however whoever used the `Hash` implementation
relied on an incorrect implementation.