py: Add hex builtin function.

A one-liner, added especially for @pfalcon :)
diff --git a/py/builtin.c b/py/builtin.c
index de1a8fc..b02608b 100644
--- a/py/builtin.c
+++ b/py/builtin.c
@@ -207,6 +207,12 @@
 
 MP_DEFINE_CONST_FUN_OBJ_1(mp_builtin_hash_obj, mp_builtin_hash);
 
+STATIC mp_obj_t mp_builtin_hex(mp_obj_t o_in) {
+    return mp_binary_op(MP_BINARY_OP_MODULO, MP_OBJ_NEW_QSTR(MP_QSTR__percent_x), o_in);
+}
+
+MP_DEFINE_CONST_FUN_OBJ_1(mp_builtin_hex_obj, mp_builtin_hex);
+
 STATIC mp_obj_t mp_builtin_iter(mp_obj_t o_in) {
     return mp_getiter(o_in);
 }