Use the Deref traits for FlowRefs.

This patch switches FlowRefs to using the Deref and DerefMut traits, instead of
the custom `get` and `get_mut` functions.
auto merge of #3686 : cgaebel/servo/deref-flows, r=pcwalton

This patch switches FlowRefs to using the Deref and DerefMut traits, instead of
the custom `get` and `get_mut` functions.