Think through and clean up various TempBlock classes

[dagss]

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.

[dagss]

TempNode? should go into this as well. It doesn't work well as it is because with a single ExprNode? temp acquisition and node evaluation is intimately linked, and this doesn't fit where TempNode? has been used.