Define equality for sets

[OrKoN]

A definition of what it means for two sets to be equal. This definition re-uses the subset/superset definitions but we could also define it as an iteration over elements and size equality.

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[sadym-chromium]

SGTM

[annevk]

I guess this works, but we're also running into #643 with these.

@domenic you okay with generally using "equal(s)" for structural equality and "is" for identity equality?

[domenic]

Yeah, this seems reasonable to me. Although it's a bit silly that we haven't defined what it means for two strings to be equal, or two numbers, or similar, so this is all on slightly shaky foundations.