Change interface SDL to adhere to spec

When only one interface is implemented, there is no &. The & is only to combine more than one interface.

src/registry/export_sdl.rs

@@ -131,10 +131,7 @@ impl Registry {
                 write!(sdl, "type {} ", name).ok();
                 if let Some(implements) = self.implements.get(name) {
                     if !implements.is_empty() {
-                        write!(sdl, "implements ").ok();
-                        for interface in implements {
-                            write!(sdl, "& {} ", interface).ok();
-                        }
+                        write!(sdl, "implements {} ", implements.iter().map(AsRef::as_ref).collect::<Vec<&str>>().join(" & ")).ok();
                     }
                 }