Convert builtin function to ast expression.

def builtin_folding(value):
    """ Convert builtin function to ast expression. """
    if isinstance(value, (type(None), bool)):
        name = str(value)
    elif value.__name__ in ("bool", "float", "int"):
        name = value.__name__ + "_"
    else:
        name = value.__name__
    return ast.Attribute(ast.Name('__builtin__', ast.Load(), None),
                         name, ast.Load())