Mutable doubly-linked list with Lwt iterators

A sequence is an object holding a list of elements which support the following operations:

• adding an element to the left or the right in time and space O(1)
• taking an element from the left or the right in time and space O(1)
• removing a previously added element from a sequence in time and space O(1)
• removing an element while the sequence is being transversed.