WIP refactoring browser checks

baby steps towards showing list of checks and storing them in a more
extensible way
Use Sum type for browser support as display.

Next steps: make display look nicer, it now shows as

{"instance":"AddEventListener"}

for instance, instead of "AddEventListener". No idea how to do this yet in
Fay (I am also still stumped on {:value true} for Bool Fay
instances. But we'll figure it out.