extract C-line handling from AddTraceback() into separate function to keep AddTraceback() available as a shared header file

use faster __Pyx_PyObject_GetAttrStr() implementation and avoid static Python string reference in AddTraceback() helper

See #1719.

repair __Pyx_CallUnboundCMethod1() macro which also needs to adapt to the new CPython FASTCALL signature in Py3.7

extend tests to cover some more internal signature cases of CPython builtins (METH_FASTCALL with and without keywords in P3.7)

Resolves note at #1765.

Extern structs may only be partially declared.

Give an error for invalid options or directives

First attempt at explicit fallthrough annotation.

Also fix module refcounting for #1764 and remove redundant variable assignment (which just triggers a useless type check).

Don't use __Pyx_PyObject_CallMethO on cyfunctions

Previously these could be suppressed if error holding was in place.

This was noticed in #1746.