fix: 'Promise.isPromise' checks metatable over reading ClassName

[OttoHatt]

I've been wanting to return Table.readOnly tables from inside promises. However I've run into an issue: every value returned from a Promise is checked against Promise.isPromise for chaining, which reads ClassName directly and causes the readonly metatable to throw. My solution is to use getmetatable instead, like newer Nevermore classes.

[Quenty]

I'm good for this.

I think we should generalize the isA() calls to be duck typing instead of this classname checks.

[Quenty]

Oh wait, this can technically break stuff because of the way Rx works with multiple copies of promises. I'm still ok with this change, but duck typing would be better. Probably need a Promiseinterface which works with eryn's promise impl. Could standardize a spec within Nevermore.

[OttoHatt]

Switching to getmetatable(x).ClassName == "Promise" may be a good enough workaround for now? Metatables typically aren't protected.

[Quenty]

I think so! We're probably going to need to dummy-type the Promise for compatibility with Eryn's promise soon anyway.

[OttoHatt]

Looks like 14874e8 added ducktyping. Worth looking at again? Not sure of the performance implications over just checking the metatable.