Collect/TryCollect does not use Stream::size_hint

[jhgg]

I was reading through the implementation of collect/try_collect, and realized that the size_hint of the stream was not being used. Is there any good reason for this, or is this just a bug/not yet implemented?

[taiki-e]

In the current API, unstable Extend::extend_reserve (rust-lang/rust#72631) is needed to use size_hint. See also #1833.