py: Rename builtin "io" to "_io".

Functionality we provide in builtin io module is fairly minimal. Some
code, including CPython stdlib, depends on more functionality. So, there's
a choice to either implement it in C, or move it _io, and let implement other
functionality in Python. 2nd choice is pursued. This setup matches CPython
too (_io is builtin, io is Python-level).

py/builtintables.c

@@ -162,7 +162,7 @@ STATIC const mp_map_elem_t mp_builtin_module_table[] = {
 
     { MP_OBJ_NEW_QSTR(MP_QSTR_array), (mp_obj_t)&mp_module_array },
 #if MICROPY_PY_IO
-    { MP_OBJ_NEW_QSTR(MP_QSTR_io), (mp_obj_t)&mp_module_io },
+    { MP_OBJ_NEW_QSTR(MP_QSTR__io), (mp_obj_t)&mp_module_io },
 #endif
 #if MICROPY_PY_COLLECTIONS
     { MP_OBJ_NEW_QSTR(MP_QSTR__collections), (mp_obj_t)&mp_module_collections },

py/modio.c

@@ -36,7 +36,7 @@ extern const mp_obj_type_t mp_type_fileio;
 extern const mp_obj_type_t mp_type_textio;
 
 STATIC const mp_map_elem_t mp_module_io_globals_table[] = {
-    { MP_OBJ_NEW_QSTR(MP_QSTR___name__), MP_OBJ_NEW_QSTR(MP_QSTR_io) },
+    { MP_OBJ_NEW_QSTR(MP_QSTR___name__), MP_OBJ_NEW_QSTR(MP_QSTR__io) },
     // Note: mp_builtin_open_obj should be defined by port, it's not
     // part of the core.
     { MP_OBJ_NEW_QSTR(MP_QSTR_open), (mp_obj_t)&mp_builtin_open_obj },
@@ -65,7 +65,7 @@ STATIC const mp_obj_dict_t mp_module_io_globals = {
 
 const mp_obj_module_t mp_module_io = {
     .base = { &mp_type_module },
-    .name = MP_QSTR_io,
+    .name = MP_QSTR__io,
     .globals = (mp_obj_dict_t*)&mp_module_io_globals,
 };
 

py/qstrdefs.h

@@ -361,7 +361,7 @@ Q(unpack)
 #endif
 
 #if MICROPY_PY_IO
-Q(io)
+Q(_io)
 Q(readall)
 Q(readline)
 Q(readlines)

tests/io/stringio1.py

-import io
+import _io as io
 
 a = io.StringIO()
 print(a.getvalue())