Add blog post about changing a Set's definition of unique

[Stuk]

No description provided.

[Stuk]

Could you review, @kriskowal?

[kriskowal]

At the moment, Set uses Object.equals, but this is probably a bug. It should use Object.is, which does referential identity, which is to say === except that Object.is(NaN, NaN) and Object.is(-0, -0). Object.equals does a fancier kind of equivalence, where object and array literals with equivalent values are equivalent, and where objects that implement an equals method on their prototype can define their own notion of equivalence.