baseExpression: Allow Identifiers as names
Add RewriteReturnsEditor

This is how value bodies could be rewritten for
ceylon/ceylon-compiler#1794 / ceylon/ceylon-compiler#1798 if it has been
determined that they contain no early returns.
Add tests for RewriteReturnEditor

CC ceylon/ceylon-compiler#1794 / ceylon/ceylon-compiler#1798.