Tuple conformance

Conformance test for tuple feature (PR #428)
heyYui, can you file a bug on having us provide a better error message here.  Here we're giving an error about 'pop' being wrong (where pop is a method on Array), where it would probably be better to indicate that the type of the tuple array isn't assignable to number.

This is all about improving the error message and providing more directed, useful, information to the person who runs into this.

Thanks!