GH-131238: Core header refactor (GH-131250)

* Moves most structs in pycore_ header files into pycore_structs.h and pycore_runtime_structs.h

* Removes many cross-header dependencies

Objects/unicodeobject.c

@@ -53,6 +53,7 @@ OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 #include "pycore_object.h"        // _PyObject_GC_TRACK(), _Py_FatalRefcountError()
 #include "pycore_pathconfig.h"    // _Py_DumpPathConfig()
 #include "pycore_pyerrors.h"      // _PyUnicodeTranslateError_Create()
+#include "pycore_pyhash.h"        // _Py_HashSecret_t
 #include "pycore_pylifecycle.h"   // _Py_SetFileSystemEncoding()
 #include "pycore_pystate.h"       // _PyInterpreterState_GET()
 #include "pycore_ucnhash.h"       // _PyUnicode_Name_CAPI