Think through and clean up various TempBlock classes
|Reported by:||dagss||Owned by:||somebody|
UtilNodes.py currently have two different TempBlock?-style classes. These fit different purposes, but are still too dissimilar etc. One unifying concept like a LetNode (or two different ones, one statement and one expression, but still a similar API) should be introduced before it is used in too many situations.
This work should ideally include getting rid of the ugly hack I'm about to commit in TreeFragment, where a full variable is allocated rather than a temporary. See #151 for details.