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.
Fix whitespace_pre with incremental reflow turned on.

This implements fragment merging, in order to incrementally reflow linebroken
text. This makes the `whitespace_pre.html` reftest pass with incremental reflow
turned on with `-i`.