Feature: Add DataFromStream

[letmutx]

DataFromReader has buffering on the HTTP response, so it is not suitable to stream data to a client. DataFromStream disables buffering and flushes data as soon as it is sent to the HTTP body.

Admittedly, we could let users control when we do the flushing, say, every N bytes. But I wanted to know if there's interest in this feature before building more things.