Fix uses of @convertible.

lib/std/core/dstring.c3

@@ -282,9 +282,9 @@ macro void DString.append(&self, value)
 			self.append_char32(value);
 		$default:
 			$switch
-				$case @convertible(value, Char32):
+				$case $assignable(value, Char32):
 					self.append_char32(value);
-				$case @convertible(value, String):
+				$case $assignable(value, String):
 					self.append_chars(value);
 				$default:
 					$error "Unsupported type for append – use printf instead.";

lib/std/io/stream.c3

@@ -136,7 +136,7 @@ macro usz! print_gen(self, x)
 	$case DString:
 		return self.write(x.str_view());
 	$default:
-		$if @convertible(x, String):
+		$if $assignable(x, String):
 			return self.write((String)x);
 		$else
 			return printf("%s", x);