py/objstr: Remove "make_qstr_if_not_already" arg from mp_obj_new_str.

This patch simplifies the str creation API to favour the common case of
creating a str object that is not forced to be interned.  To force
interning of a new str the new mp_obj_new_str_via_qstr function is added,
and should only be used if warranted.

Apart from simplifying the mp_obj_new_str function (and making it have the
same signature as mp_obj_new_bytes), this patch also reduces code size by a
bit (-16 bytes for bare-arm and roughly -40 bytes on the bare-metal archs).

py/builtinimport.c

@@ -434,7 +434,7 @@ mp_obj_t mp_builtin___import__(size_t n_args, const mp_obj_t *args) {
                     DEBUG_printf("%.*s is dir\n", vstr_len(&path), vstr_str(&path));
                     // https://docs.python.org/3/reference/import.html
                     // "Specifically, any module that contains a __path__ attribute is considered a package."
-                    mp_store_attr(module_obj, MP_QSTR___path__, mp_obj_new_str(vstr_str(&path), vstr_len(&path), false));
+                    mp_store_attr(module_obj, MP_QSTR___path__, mp_obj_new_str(vstr_str(&path), vstr_len(&path)));
                     size_t orig_path_len = path.len;
                     vstr_add_char(&path, PATH_SEP_CHAR);
                     vstr_add_str(&path, "__init__.py");