added example code and stub impl files
more stubbing
comments
WIP
basic working asynchronous iteration
Added exception safety, need better comments/docs
working fs tree walking example
reverted to old await()

newly optimised one breaks on largest() benchmark
experimental forEach() on iterator
experimental: largest-asyncawait using iterators

but it is slower than before
added comparative benchmark
refactor (no speedup)
slight speed improvement

by yielding in batches (so yield has non-negl overhead)
improved concurrent performance

but still leaks memory and gradually slows down....