bpo-42882: _PyRuntimeState_Init() leaves unicode next_index unchanged (GH-24193)

Fix the _PyUnicode_FromId() function (_Py_IDENTIFIER(var) API) when
Py_Initialize() / Py_Finalize() is called multiple times:
preserve _PyRuntime.unicode_ids.next_index value.

Use _PyRuntimeState_INIT macro instead memset(0) to reset
_PyRuntimeState members to zero.