Don't check the field parameter in LayoutNode::eq.

The chain field shouldn't be relevant for equality, since it is just a
hack to artificially extend lifetimes.
Use ContravariantLifetime in LayoutNode.

Since ContravariantLifetime uses no storage, this will reduce the size
of a LayoutNode from 3 words to 2.
auto merge of #2436 : zwarich/servo/contravariant-lifetime-layout, r=pcwalton

Since ContravariantLifetime uses no storage, this will reduce the size of a LayoutNode from 3 words to 2.