Improve isCID?

[alanshaw]

isCID checks version, codec and multihash properties are present and validates them. This proves that the passed object is an object that looks like { version, codec, multihash } and each property is valid, but doesn't actually prove that the passed object is an instance of CID.

So you could get back a true from isCID but there's a possibility that toBaseEncodedString (and the other instance methods) don't exist!

I know it's not the perfect solution, but would using https://github.com/moxystudio/js-class-is be better?

It's still possible to "fake" a CID instance, but it's much harder to do it by accident and we might even get a performance win if we make version, codec, and multihash readonly, because we can safely assume they are valid if we can determine the passed object is an instance of CID.

[mikeal]

I actually assumed this was already the case because I'd seen is-class used in other related modules. Big +1.