Issue #695: Automatically export C helper functions

The pythonapi.fix_python_api function exports C helper functions.
However, each export is done manually by adding a new call to
llvm.ee.dylib_add_symbol for the appropriate helper defined in
_helperlib.c_helpers.  This commit automates the process by just
exporting *every* symbol defined in _helperlib.c_helpers.
Merge pull request #703 from meadori/issues/695

Issue #695: Automatically export C helper functions