codal_port/modmicrobit: Put audio module as entry in microbit module.

On micro:bit v2 the audio functionality is "built in", similar to the
display functionality.  And now the following is possible and nicely
consistent:

    from microbit import *
    display.show(Image.HAPPY)
    audio.play(Sound.HAPPY)

The "music" and "speech" modules are separate modules because they
provide functionality on top of the base audio functionality.

Author: dpgeorge

src/codal_port/modmicrobit.c

@@ -104,6 +104,8 @@ STATIC const mp_rom_map_elem_t microbit_module_globals_table[] = {
     { MP_ROM_QSTR(MP_QSTR_accelerometer), MP_ROM_PTR(&microbit_accelerometer_obj) },
     { MP_ROM_QSTR(MP_QSTR_compass), MP_ROM_PTR(&microbit_compass_obj) },
     { MP_ROM_QSTR(MP_QSTR_microphone), MP_ROM_PTR(&microbit_microphone_obj) },
+    { MP_ROM_QSTR(MP_QSTR_audio), MP_ROM_PTR(&audio_module) },
+
     { MP_ROM_QSTR(MP_QSTR_i2c), MP_ROM_PTR(&microbit_i2c_obj) },
     { MP_ROM_QSTR(MP_QSTR_uart), MP_ROM_PTR(&microbit_uart_obj) },
     { MP_ROM_QSTR(MP_QSTR_spi), MP_ROM_PTR(&microbit_spi_obj) },