change [draw group] to [draw g] to match svg spec name for groups

This changes [draw group] to [draw g] for the new data structure drawings. It's a pretty disruptive change in the interface, but a very easy one to implement. So I wanted to make it before hitting a release candidate, and before too many people begin using data structures.

This way the main [draw] arguments are exactly the same name as their svg counterparts. It's a fairly small usability improvement-- the user can go to the spec or tutorial for SVGs and immediately know that <g> corresponds to [draw g]. Still, I think that consistency is important and helpful, so I wanted to fix it before releasing.