wireproto: start to associate frame generation with a stream
An upcoming commit will introduce "streams" into the frame-based wire
protocol. In preparation for this invasive change, we introduce a basic
"stream" class and have all operations that create frames also operate
alongside a stream instance.
Differential Revision: https://phab.mercurial-scm.org/D2906
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors