Make VertexAttributes implement Iterable

Closes #1228
Basically includes a copy of Array.ArrayIterable (ReadonlyIterable) and Array.ArrayIterator (ReadonlyIterator), but with a backing fixed array T[] instead of an Array<T> instance. Currently I made them subclasses of the VertexAttributes class, but it's probably better to rename and move them. If you agree, let me know (including a better name) and I will change it accordingly.

Also a minor change: the checkValidity() method is only needed for GLES1.x, so changed the name and only call it when isGL20Available() returns false.