Returns a pair [edit-distance edit-script] as result of comparision
of sequences as and bs.

Edit-distance is an integer.

The edit-script is a sequence of vectors starting with an insert or
delete operation symbol :+ or :-.

An insert is [:+ position items].
A delete is  [:- position number-of-items].

The edit-script is made for sequential processing with operations
like insert-at: [xs pos items -> xs'] and  remove-at: [xs pos n -> xs'].

Insert sequence ys at position i into xs.

Applies the edit-script (as contained in the result of diff) to
sequence as, using by default insert-at and remove-at as implemented
in this namespace. Returns a vector.

Remove n items at position i from xs.