h2::Error now knows whether protocol errors happened because the user sent them, because it was received from the remote peer, or because the library itself emitted an error because it detected a protocol violation. It also keeps track of whether it came from a RST_STREAM or GO_AWAY frame, and in the case of the latter, it includes the additional debug data if any. Fixes #530
h2 test support
This crate includes utilities for writing h2 tests. This is broken up into a
separate crate because it requires the unstable feature flag and to enable
tower-h2 to use the same helpers.