Here's a second patch; I think this is ready to go in

I cleaned up the node parsing a lot.  It now knows how to parse the following types of default values:
  * Number (this applies to both ints and floats)
  * String ('hello')
  * Attribute (sys.maxsize)
  * Named constant (None, True, False)
As long as I'm fixing this, I want to add support for any other values we're going to want.  Did I miss any?  Note that we can't really support container types, because initializing them on the C side isn't practical.

Also, the tests don't exercise all of the above possible node types, just because I couldn't remember builtins that used those types.  We'll fill in the remaining tests as we port new functions to Argument Clinic.

If I can get a quick LGTM review, I'll check this in.