bpo-46008: Make runtime-global object/type lifecycle functions and state consistent. (gh-29998)

This change is strictly renames and moving code around.  It helps in the following ways:

* ensures type-related init functions focus strictly on one of the three aspects (state, objects, types)
* passes in PyInterpreterState * to all those functions, simplifying work on moving types/objects/state to the interpreter
* consistent naming conventions help make what's going on more clear
* keeping API related to a type in the corresponding header file makes it more obvious where to look for it

https://bugs.python.org/issue46008

Include/internal/pycore_structseq.h

@@ -9,6 +9,13 @@ extern "C" {
 #endif
 
 
+/* runtime lifecycle */
+
+extern PyStatus _PyStructSequence_InitState(PyInterpreterState *);
+
+
+/* other API */
+
 PyAPI_FUNC(int) _PyStructSequence_InitType(
     PyTypeObject *type,
     PyStructSequence_Desc *desc,