bpo-34170: Add Python/coreconfig.c for _PyCoreConfig (GH-8607)

* Add Include/coreconfig.h
* Move config_*() and _PyCoreConfig_*() functions from Modules/main.c
  to a new Python/coreconfig.c file.
* Inline _Py_ReadHashSeed() into config_init_hash_seed()
* Move global configuration variables to coreconfig.c