bpo-40521: Optimize PyBytes_FromStringAndSize(str, 0) (GH-21142)

Always create the empty bytes string singleton.

Optimize PyBytes_FromStringAndSize(str, 0): it no longer has to check
if the empty string singleton was created or not, it is always
available.

Add functions:

* _PyBytes_Init()
* bytes_get_empty(), bytes_new_empty()
* bytes_create_empty_string_singleton()
* unicode_create_empty_string_singleton()

_Py_unicode_state: rename empty structure member to empty_string.