esp8266: Convert mp_hal_pin_obj_t from pin ptr to simple integer.

Most pin I/O can be done just knowing the pin number as a simple
integer, and it's more efficient this way (code size, speed) because it
doesn't require a memory lookup to get the pin id from the pin object.

If the full pin object is needed then it can be easily looked up in the
pin table.
diff --git a/esp8266/modpyb.h b/esp8266/modpyb.h
index 8d27ebe..c1a0413 100644
--- a/esp8266/modpyb.h
+++ b/esp8266/modpyb.h
@@ -18,6 +18,8 @@
     uint32_t periph;
 } pyb_pin_obj_t;
 
+const pyb_pin_obj_t pyb_pin_obj[16 + 1];
+
 void pin_init0(void);
 void pin_intr_handler_iram(void *arg);
 void pin_intr_handler(uint32_t);