Finish drafting CSS4 Grid support!

Draft grid width-sizing code.

Test & fix block layout.

Draft flow layout computation.

calc() interpretor & multidirectional text to be added later.

Draft new CatTrap API which hopefully is easier to implement.

Unit test calc() implementation!

Finish drafting shunting yard interpreter for calc().

Draft shunting yard interpreter for calc().

Draft public API for CatTrap.