py/obj: Add MICROPY_OBJ_IMMEDIATE_OBJS option to reduce code size.

This option (enabled by default for object representation A, B, C) makes
None/False/True objects immediate objects, ie they are no longer a concrete
object in ROM but are rather just values, eg None=0x6 for representation A.

Doing this saves a considerable amount of code size, due to these objects
being widely used:

   bare-arm:  -392 -0.591%
minimal x86:  -252 -0.170% [incl +52(data)]
   unix x64:  -624 -0.125% [incl -128(data)]
unix nanbox:    +0 +0.000%
      stm32: -1940 -0.510% PYBV10
     cc3200: -1216 -0.659%
    esp8266:  -404 -0.062% GENERIC
      esp32:  -732 -0.064% GENERIC[incl +48(data)]
        nrf:  -988 -0.675% pca10040
       samd:  -564 -0.556% ADAFRUIT_ITSYBITSY_M4_EXPRESS

Thanks go to @Jongy aka Yonatan Goldschmidt for the idea.

py/obj.c

@@ -46,6 +46,11 @@ const mp_obj_type_t *mp_obj_get_type(mp_const_obj_t o_in) {
     } else if (mp_obj_is_float(o_in)) {
         return &mp_type_float;
     #endif
+    #if MICROPY_OBJ_IMMEDIATE_OBJS
+    } else if (mp_obj_is_immediate_obj(o_in)) {
+        static const mp_obj_type_t *const types[2] = {&mp_type_NoneType, &mp_type_bool};
+        return types[MP_OBJ_IMMEDIATE_OBJ_VALUE(o_in) & 1];
+    #endif
     } else {
         const mp_obj_base_t *o = MP_OBJ_TO_PTR(o_in);
         return o->type;