xilem_web: Add MemoizedStream view

[flosse]

The MemoizedStream view is is basically a MemoizedAwait view that accepts a stream and invokes the callback for each stream item received. Since the implementation is almost the same, the two views are just wrappers for the private MemoizedInner struct. If the approach is okay, I can also add an example.

[flosse]

(Ideally we would also have an example showing a use-case for this, but I don't want to block on that...)

Of course, that will come. Unfortunately, this week looks very full for me, but apart from myself, probably nobody is waiting at the moment :wink: