Add declarations for PyObject_GenericGetAttr and PyObject_GenericSetAttr

This brings the overhead to about 10%, or 20ns per traceback.

use faster __Pyx_PyObject_GetAttrStr() implementation and avoid stati…

allow enabling/disabling C line display in tracebacks with a new C compile time macro CYTHON_CLINE_IN_TRACEBACK=0/1

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