Further Assignment Operator Fixes

Support operator bool() for C++ classes.

http://bugs.python.org/issue24619

Formerly bad code involving unassigned reference temporaries was generated.

Now that we split CInitCode and CppInitCode, we no longer generate
valid C code even if C++ features are not used.

Were these ever used, this would have been an error.

This not only fixes bugs like auto-conversion of std::map[enum, enum] but also
fixes a bug where the compiler-chosen type of int for the enum may not have been
long.

This is a bit of a hack, but as Cython doesn't yet understand
const overloads with different (usually const) return types,
it can't provide the right type of reference in this case.

generate_assignment_code signatures so that subclassing will work properly
when first building Cython.

all left hand sides have the same type.

operators declared for C++ classes.

a different type using an overloaded assignment operator.