…st autobahn

Autobahn (http://autobahn.ws/testsuite) is a comprehensive test suite
of the Websocket specifications.
This test won't be run by default as it requires way too much ressources
to put it into the CI, but can trivially be run manually by anyone wanting
to test the websockets implementation for conformity.

We should not assume what the user is sending, as it varies from one application to another.
The default might as well be binary and would seem more sensible to some.

Reduce the need for additional buffer, opening the way
for more memory improvements, and document what it is
doing.

Also corrects a bug with length == 65536, as it would be cast
to `ushort` which maximum size is 65535.

Finally, fixup documentation and type of FrameOpcode.

Reduce number of static buffers (3 -> 1)
Comment the internals
Only demask when the frame is masked (the previous code didn't yield incorrect data,
but needlessly iterated and assigned data to an array)
Check that the most significant bit of the 8 bytes length is 0.

…testsuite + improvements