Add persistent vector, set, and map

[sbohmann]

These provide all the safety of immutable collections without copying overhead when used imperatively.
Changes are made by creating modified copies that share all unaffected data with the original.
For immutable vectors this holds true for appending and removing at the end, as well as for changing values by index.
For immutable maps and sets, it holds true for all affected operations.
There us usually* an overhead involved but it is O(1).
*) unless the mutable equivalent is poorly implemented, which can easily make a reasonable persistent counterpart perform better even in imperative use with lots of changes.