Add objtuple.h to allow embedding of tuples inside other objects.

This is useful because tuple is closest analog of C static array.

py/objtuple.h

@@ -0,0 +1,7 @@
+typedef struct _mp_obj_tuple_t {
+    mp_obj_base_t base;
+    machine_uint_t len;
+    mp_obj_t items[];
+} mp_obj_tuple_t;
+
+void tuple_print(void (*print)(void *env, const char *fmt, ...), void *env, mp_obj_t o_in);