Define set difference

[annevk]

What is the issue with the Infra Standard?

For the Sanitizer API it might make sense to define the difference between two sets.

There is one caveat however which is that these sets contain IDL dictionaries. I.e., maps. The maps in question have two keys that map to either a string or null. But we don't really define equality for such more complex structures currently.

So maybe this is a bunch of custom logic the Sanitizer API will have to write out for all the dictionary manipulation it does.

cc @domenic @otherdaniel @mozfreddyb

See also #326.