Commits on Jul 20, 2020

1. Add an implementation of the core float and clear placement logic in … …

   …layout
   
   2020, not yet wired to the rest of layout.
   
   This commit implements an object that handles the 10 rules in CSS 2.1:
   
   https://www.w3.org/TR/CSS2/visuren.html#float-position
   
   The implementation strategy is that of a persistent balanced binary search tree
   of float bands. Binary search trees are commonly used for implementing float
   positioning; e.g. by WebKit.  Persistence enables each object that interacts
   with floats to efficiently contain a snapshot of the float list at the time
   that object was laid out. That way, incremental layout can invalidate and start
   reflow at any point in a containing block.
   
   This commit features extensive use of
   [QuickCheck](https://github.com/BurntSushi/quickcheck) to ensure that the rules
   of the CSS specification are followed.
   
   Because this is not yet connected to layout, floats will not actually be laid
   out in Web pages yet.
   
   Note that unit tests as set up in Servo currently require types that they
   access to be public. Therefore, some internal layout 2020 types that were
   previously private have been made public. This is somewhat unfortunate.
   
   Part of #25167.

   

   pcwalton committed Jul 20, 2020

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