extmod/machine_signal: Add signal_print() as repr() function.

Test code is:
```
from machine import Pin, Signal
signal22= Signal(Pin(22, mode=Pin.OUT))
signal22
signal21= Signal(Pin(21, mode=Pin.IN), invert=True)
signal21
```
Ounput is:
```
Signal(Pin(22))
Signal(Pin(21), invert=True)
```
instead of
```
<Signal>
<Signal>
```

extmod/machine_signal.c

@@ -114,6 +114,21 @@ STATIC mp_obj_t signal_make_new(const mp_obj_type_t *type, size_t n_args, size_t
     return MP_OBJ_FROM_PTR(o);
 }
 
+STATIC void signal_print(const mp_print_t *print, mp_obj_t self_in, mp_print_kind_t kind) {
+    machine_signal_t *self = (machine_signal_t *)self_in;
+    const mp_print_fun_t pin_print_function = (mp_print_fun_t)MP_OBJ_TYPE_GET_SLOT_OR_NULL(mp_obj_get_type(self->pin), print);
+    mp_printf(print, "Signal(");
+    if (pin_print_function) {
+        pin_print_function(print, self->pin, kind);
+    } else {
+        mp_printf(print, "Pin()"); // correct the port
+    }
+    if (self->invert) {
+        mp_printf(print, ", invert=True");
+    }
+    mp_printf(print, ")");
+}
+
 STATIC mp_uint_t signal_ioctl(mp_obj_t self_in, mp_uint_t request, uintptr_t arg, int *errcode) {
     (void)errcode;
     machine_signal_t *self = MP_OBJ_TO_PTR(self_in);
@@ -177,6 +192,7 @@ MP_DEFINE_CONST_OBJ_TYPE(
     MP_QSTR_Signal,
     MP_TYPE_FLAG_NONE,
     make_new, signal_make_new,
+    print, signal_print,
     call, signal_call,
     protocol, &signal_pin_p,
     locals_dict, &signal_locals_dict