Make Stream[T] implement AsyncIterable[T] too

[ebonnal]

An AsyncIterable[T] implements __aiter__(self) -> AsyncIterator[T]
An AsyncIterator[T] implements __anext__(self) -> T

it allows async for _ in aiterable

expected TODO:

• Implement first async alternatives to all operation having a potentially expensive function parameter (we already have .amap/.aforeach, we need afilter...)
• Overload the constructor: __init__(self, source: AsyncIterable[T])
• Implement a AsyncIteratorVisitor(Visitor[AsyncIterator[T]]) that raises if non async source or non async function is involved in the stream