Rename LazyXXX.flush() to LazyXXX.commit(), make LazyXXX.commit() write itself to...

Rename LazyXXX.flush() to LazyXXX.commit(), make LazyXXX.commit() write itself to disk instead of relying on parent to do so (because there is no longer any need for special handling of the root object to put < > around it since the < > will now surround a [ptr, len] record rather than the root object itself), fix semantics of LazyArray.delete() and LazyArray.pop() so length behaves like Array