Message serialisation for the Marionette protocol was largely moved
into a separate Rust crate, marionette, as part of bug 1569100
and preceding bugs, but geckodriver still maintains control over
the TCP connection it establishes to the Marionette server in
Firefox.

This bug tracks creating a session API in the marionette crate
so that implementation details about the protocol are not leaked
to geckodriver. The end goal is to make the Marionette backend in
geckodriver pluggable so that it can be replaced with alternate
backends, e.g. for CDP.

Lets not do it before bug 1525126 has been finished. It adds a certain amount of new code.