objstrunicode: Re-add buffer protocol back for now, required for io.StringIO.

py/objstr.c

@@ -1601,7 +1601,7 @@ STATIC mp_obj_t str_encode(uint n_args, const mp_obj_t *args) {
 }
 #endif
 
-STATIC machine_int_t str_get_buffer(mp_obj_t self_in, mp_buffer_info_t *bufinfo, int flags) {
+machine_int_t str_get_buffer(mp_obj_t self_in, mp_buffer_info_t *bufinfo, int flags) {
     if (flags == MP_BUFFER_READ) {
         GET_STR_DATA_LEN(self_in, str_data, str_len);
         bufinfo->buf = (void*)str_data;

py/objstr.h

@@ -55,6 +55,7 @@ mp_obj_t mp_obj_str_format(uint n_args, const mp_obj_t *args);
 mp_obj_t mp_obj_new_str_of_type(const mp_obj_type_t *type, const byte* data, uint len);
 
 mp_obj_t str_binary_op(int op, mp_obj_t lhs_in, mp_obj_t rhs_in);
+machine_int_t str_get_buffer(mp_obj_t self_in, mp_buffer_info_t *bufinfo, int flags);
 
 MP_DECLARE_CONST_FUN_OBJ(str_encode_obj);
 MP_DECLARE_CONST_FUN_OBJ(str_find_obj);

py/objstrunicode.c

@@ -313,7 +313,7 @@ const mp_obj_type_t mp_type_str = {
     .binary_op = str_binary_op,
     .subscr = str_subscr,
     .getiter = mp_obj_new_str_iterator,
-//    .buffer_p = { .get_buffer = str_get_buffer },
+    .buffer_p = { .get_buffer = str_get_buffer },
     .locals_dict = (mp_obj_t)&str_locals_dict,
 };