Rewrite flow construction to be incrementalizable and parallelizable.

This replaces flow construction with a strict bottom-up tree traversal,
allowing for parallelism. Each step of the traversal creates a flow or
a `ConstructionItem`, similar to how Gecko works. {ib} splits are
handled by not creating `InlineFlow`s until the containing block is
reached.

This should be able to be incrementalized by storing the `Flow` from
layout to layout, and performing fixups during flow construction
and/or wiping containing blocks in a previous pass.

r? @larsbergstrom