`stream_writable` for new streams

I created a wrapper around each stream that calls `stream_writable` before `stream_send`. The idea is that this would queue up `stream_writable_next` if false. Unfortunately this doesn't work for new streams, as `stream_writable` returns `InvalidStreamState` if the stream has not been created yet.

This feels counter-intuitive because the `stream_send` will succeed; the stream is in fact writable. I think it would be useful behavior because it removes the guess-work for `peer_streams_left`. The new stream would only be marked writable (createable) when MAX_STREAMS flow control allows it.

My workaround is to call `stream_send(id, &[], false)` first to create the empty stream first. In hind-sight, I should really just try calling `stream_send` first and `stream_writable` only when it performs an incomplete write, but it would still be useful for MAX_STREAMS.

Start by tracing the stream_writable and stream_send entry points for streams that have not yet been created, focusing on MAX_STREAMS flow control and the InvalidStreamState result. Compare the proposed writable/createable behavior with stream_writable_next and verify that existing stream_send behavior remains unchanged.

Written by the indexing model from the issue text.