use pre-instantiated identifier string when looking up or setting the vtable of exported/imported extension types

handle some more cases where the error status is known to be used and clean up 'unraisable exception' code

Autodoc improvements

Regression test for #42.

Add strspn to string.pxd

Fixed .srctree tests execution

fix compiler crash on bad code (complex % x) and move directive handling in ModNode to analysis phase

Removed the warning about docstrings for private attributes. Fixed erroneous docstring recognition past the blank line.