ast: Remove unnecessary over-setting of result to NULL.
Compound operations now work for instance values. Yay!

They still don't work for package values though. :(
Finally make package assign account for compound ops. Fixes #40.

I tested this by adding an integer value to sys, but then taking it out since
the value was not actually necessary. Doing a += with that value worked, so
I'm calling this working.

Additionally, package_{set/get}_deep are both gone now, because those haven't
been used in a long time (ever?) and I didn't feel like fixing broken code.
Good riddance.