layout: Implement the correct behavior for block formatting contexts adjacent to floats per CSS 2.1 § 9.5.

Results in an 80% parallel improvement over sequential on youtube.com.

This is quite tricky—it involves a sea change to layout in order to make block layout interruptible and resumable, and intertwines assign-widths and assign-heights even more—and I'd be happy to go over this via video or IRC.

r? @larsbergstrom