Improve iface error message when types differ in boxity

[catamorphism]

I just got the error failed to find an implementation of interface middle::typeck::ast_conv for middle::typeck::crate_ctxt. It took me a second to figure out that my mistake was leaving off the @ on crate_ctxt. It would be nice if, in this case, the typechecker gave a hint like "did you mean @crate_ctxt?", in the case where there is an implementation of ast_conv for @crate_ctxt.

[pcwalton]

How does this interact with issue #2212?

[catamorphism]

I think it's basically independent from #2212 ? Regardless of what behavior gets decided on for #2212, I'd think it should still be possible for the typechecker to give hints in some obvious cases (obviously it won't be perfect).

[catamorphism]

Hmm, I don't know what I was thinking. This seems like no longer an issue with #2212 being implemented. I can't think of a case where it does come up now. Closing.

[brson]

I like the term 'boxity'