extmod/machine_signal: Support all Pin's arguments to the constructor.

This implements the orginal idea is that Signal is a subclass of Pin, and
thus can accept all the same argument as Pin, and additionally, "inverted"
param. On the practical side, it allows to avoid many enclosed parenses for
a typical declararion, e.g. for Zephyr:

Signal(Pin(("GPIO_0", 1))).

Of course, passing a Pin to Signal constructor is still supported and is the
most generic form (e.g. Unix port will only support such form, as it doesn't
have "builtin" Pins), what's introduces here is just practical readability
optimization.

"value" kwarg is treated as applying to a Signal (i.e. accounts for possible
inversion).

extmod/virtpin.h

@@ -38,3 +38,6 @@ typedef struct _mp_pin_p_t {
 
 int mp_virtual_pin_read(mp_obj_t pin);
 void mp_virtual_pin_write(mp_obj_t pin, int value);
+
+// If a port exposes a Pin object, it's constructor should be like this
+mp_obj_t mp_pin_make_new(const mp_obj_type_t *type, size_t n_args, size_t n_kw, const mp_obj_t *args);