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