Default values are no longer generated as Python code by default.
They must be explicitly turned on using the "python:defaultargs" feature.
Closes#294Closes#296
The problems in these two issues when "python:defaultargs" is turned
on still need to be fixed and should be addressed in separate patches.
The important thing is the default code generation is now fixed.
Enum values are just (integer) constants in Python and so can be used as the
function default values just as well as literal numbers, account for this when
checking whether function parameters can be represented in Python.
Also rename is_primitive_defaultargs() to is_representable_as_pyargs() to
describe better what this function does.
