Classes made from Struct should have default new singleton method.

Co-authored-by: Yusuke Endoh <mame@ruby-lang.org>
Co-authored-by: John Hawthorn <john@hawthorn.email>
Co-authored-by: Adam Hess <HParker@github.com>
Co-authored-by: Jose Cortinas <jacortinas@gmail.com>
Co-authored-by: Jean Boussier <jean.boussier@gmail.com>