Fix "NoneType object is not callable" exception when using a tracing function that returns None (#1769)

CPython uses the return value of tracing function to assign it to frame.f_trace field and then it uses it for the frame instead of tstate.c_tracefunc. If it is None, the context shouldn't be traced line by line.

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.