Make PropertyDeclarationBlock have Arcs.
s/FlowContext/Flow

This patch is for:
https://github.com/mozilla/servo/issues/1282
auto merge of #1311 : pradeep90/servo/rule-hash-optimize, r=pcwalton

Commit for #1301
auto merge of #1326 : brunoabinader/servo/rename-flowcontext, r=pcwalton

This patch is for:
https://github.com/mozilla/servo/issues/1282
layout: Change `RenderBox` to an enum and shorten its name in
preparation for removing its `@`-ness.

Also removes a few text copies that were taking place.

This sure does remove a lot of code!