expanding_array -- an array with a length and a (possible larger) capacity

An expanding array is a persistent data structure that has cumulative O(1)
performance of adding single elements at its end.

WARNING: Due to the high worst-case time for addition, this structure should
not be used in situations when adding a single element repeatedly to the same
instance of `expanding_array`. If the resulting
`expanding_array`'s length is `l`, this will trigger the worst-case
addition time, resulting in cumulative time O(m*l) for adding an element m
times.

This constructor is for internal use only, to create instance of
`expanding_array`, use `(expanding_array T).empty` to create an empty
expanding array instance.