Implement spliterator for both nodes and edges

[mbastian]

• Node/Edge Iterable now have a spliterator(), stream() and parallel() stream methods.
• Spliterator is implemented in Node/EdgeStore by splitting over the backing arrays. Those spliterator aren't using the auto read locking mechanism, like the traditional iterators, but would throw an exception if the structure is concurrently modified
• Iterable wrappers fallback in using iterator and spliterator aren't available
• Wrapped iterators (undirected, type, or other predicates) are using SIZED when possible, but not SUBSIZED. Some iterators like self-loop don't have SIZED as we don't maintain the number of selfloops.