[devmgr] Use simple C bindings for fuchsia.crash

* Rather than encoding the message manually, devmgr now uses the simple
C bindings to send the crash message.

* Add a test that the simple C bindings work.

* Improve the generator to work for messages that contain handles. In
principle, we could use less stack if we computed the upper bound on
the number of handles in a message, but we don't do that yet.

Test: fidl-simple
Change-Id: I4630449e2d028a918107d0045810125d9c2decd6