New cs173.plt

We have developed a new version of cs173.plt that now has two language levels — the previous beginner level, plus a new advanced level that allows you to use all of DrScheme's features (including continuations). The old version is still available as old-cs173.plt.

Note: You'll have to delete the exisiting collection before you install the new one. The directory to remove is $PLTHOME/collects/cs173.