Introduce OutBodyStreamingUnmask

[edsko]

This is a new attempt at providing an unmask callback to streaming requests, replacing #77.

As per the discussion in #77, I've new introduced OutBodyStreamingUnmask alongside OutBodyStreaming.

This was actually a useful change. When I first wrote #77, I didn't really understand the server part of http2 very well yet, as I had not yet implemented the server part of my gRPC library. I now have, and understand the server aspect of http2 much better. My suspicion in #77 that I might have gotten the control flow wrong for the server API turned out to be correct: the point of this change is that user code gets control over when exceptions are unmasked in newly spawned threads. On the client side, that newly spawned thread is created in sendRequest (in Network.HTTP2.Client.Run). On the server-side, however, this works a bit differently: http2 spawns a new thread for each incoming request (call to setAction in run, Network.HTTP2.Server.Run); it then does not spawn a new thread for a streaming response body.

Therefore, this PR only deals with the client side, and OutBodyStreamingUnmask is not actually supported at all on the server side (this PR introduces requestStreamingUnmask, without a corresponding responseStreamingUnmask). It might be useful to change the Server type synonym so that the server action passed to run would also be given an unmask callback, but that has now become orthogonal to this PR.

[kazu-yamamoto]

Beautiful!

[kazu-yamamoto]

Merged.
Thank you!