Add tests for the new Iterable.string implementation
Add tests for the new Iterable.string implementation

Tests that it still returns the same string, and that it only evaluates
exactly as many elements as necessary.

lucaswerkmeister/ceylon.language,ceylon/ceylon.language,unratito/ceylon.language,ceylon/ceylon.language,unratito/ceylon.language,jvasileff/ceylon.language,jvasileff/ceylon.language,lucaswerkmeister/ceylon.language