remove methods

[KaranJain21]

Adds the following methods

• checked_remove: checks if the element at a certain index can be removed from the vector, then removes it, shifting all elements after it to the left. If it is not possible to remove, returns a RemoveError
• remove: calls checked_remove, then unwraps
• checked_swap_remove: checks if the element at a certain index can be removed from the vector, then removes it, replacing it with the last element. If it is not possible to remove, returns a RemoveError
• swap_remove: calls checked_swap_remove, then unwraps

All of the above methods return the removed element (if any).

Also add a RemoveError enum, with two variants:

1. LengthOne: to avoid removing from a NonEmpty of length 1
2. IndexOutOfBounds: for index larger than or equal to the length of the NonEmpty

[FintanH]

I'm thinking about what the correct API for this is. What I'm trying to keep in mind is that the non-emptiness of the collection is what we're always trying to maintain.

With that in mind, I think I'd prefer something like:

/// Removes the element found at `index`.
///
/// If the index exists, then the element is returned, alongside the other elements as a `Vec<T>`.
///
/// If the index is out of bounds then the `NonEmpty<T>` is returned as-is.
pub fn remove(self, index: usize) -> Result<(T, Vec<T>), NonEmpty<T>> {
  /* index shenanigans */
}

Note that we cannot guarantee that we have a non-empty collection as soon as we remove, so the caller can call NonEmpty::from_vec, and have their own error types as needed.

For swap_remove, it would be something similar:

pub fn swap_remove(self, index: usize) -> Result<NonEmpty<T>, NonEmpty<T>> {
  /* index shenanigans */
}

Noting here that we can actually return a NonEmpty because we're maintaining the non-emptiness 😁

How do you feel about this? Would you be up for trying out a version of the PR that has this API?