Partial support for %constant and structs

Test case is slightly modified from the test case in issue #250

Use of constant objects does not seem to work in Python - the type is
SwigPyObject instead of constant_directive.Type1.
fixed python global object constants

Fix for Python and -builtin
Fix from Github issue #250
Improve Python builtin and %constant structs

Tweak to previous commit from issue #250 for C compatibility.
Set self to zero too.