Clean up Static Python vectorcall entry points

Summary:
* Fix the type of `Ci_PyFunction_CallStatic` to match `vectorcallfunc`.

* Comment what the difference between `Ci_StaticFunction_Vectorcall` and
  `Ci_PyFunction_CallStatic` is.

* Remove a few conditionals or replace with ternaries. It means adding spurious
  `(void)foo;` statements to avoid unused variable warnings, but leads to more
  straight-line code.

* Match the order the functions are declared and defined in.

Reviewed By: DinoV

Differential Revision: D67875121

fbshipit-source-id: d0cfc70432ad470e8663d525dc9607731d11cf2e

Include/eval.h

@@ -24,7 +24,7 @@ CiAPI_DATA(int) _PyEval_LazyImportsEnabled;
 
 CiAPI_FUNC(PyObject *) Ci_PyFunction_CallStatic(PyFunctionObject *func,
                                  PyObject* const* args,
-                                 Py_ssize_t nargsf,
+                                 size_t nargsf,
                                  PyObject *kwnames);
 
 #endif