Multiple return values in Core

Regardless of if we want #272, it would be nice to have multiple return values starting from Core. Optimizations could then generate such functions. This is only relevant for statements, as impure extern definitions and pure extern definitions don't need that features for now.

Start by locating the Core representation and compilation path for statements, then compare how pure and impure extern definitions are handled. Review the relationship to issue #272 before choosing an approach. Done means Core statements support multiple return values while both kinds of extern definitions remain unchanged for now.

Written by the indexing model from the issue text.