Replace iterables throughout

[wemeetagain]

Using the iterable interface instead of Arrays is much slower, even a for of of an array is slower than a simple for(;;) iterating over an array. Since ssz is a low level library, meant to be used in higher-performance applications (such as Lodestar), we need to squeeze as much performance as possible.

There's still some iterator nonsense in ssz that should be removed:

• use of iterateAtDepth, not to be confused with Tree#iterateNodesAtDepth which is resolved with #167, needs a getGindicesAtDepth alternative
• use of generator functions in public interface, needs an array alternative, can be introduced in a non-breaking change