py/obj: Convert make_new into a mp_obj_type_t slot.

Instead of being an explicit field, it's now a slot like all the other
methods.

This is a marginal code size improvement because most types have a make_new
(100/138 on PYBV11), however it improves consistency in how types are
declared, removing the special case for make_new.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>
diff --git a/ports/esp32/machine_hw_spi.c b/ports/esp32/machine_hw_spi.c
index 51ea31a..05b1c87 100644
--- a/ports/esp32/machine_hw_spi.c
+++ b/ports/esp32/machine_hw_spi.c
@@ -549,7 +549,7 @@
     machine_hw_spi_type,
     MP_QSTR_SPI,
     MP_TYPE_FLAG_NONE,
-    machine_hw_spi_make_new,
+    make_new, machine_hw_spi_make_new,
     print, machine_hw_spi_print,
     protocol, &machine_hw_spi_p,
     locals_dict, &mp_machine_spi_locals_dict