Migration improvements: insert missing assignments

[sjbreiner]

I added a preprocessing step to insert generators that are not assigned in a migration. I also added new syntactic sugar for maps to/from a @unit/@empty object

• If a missing generator f is a HomExpr and its target is a unit object, then attach a new line to the body f => @delete
• If a missing generator f is a HomExpr and its source is an empty object, then attach a new line f => @zero
• For any other missing generator X, attach a new line X => X
• Add a substitution step before calling parse_query_hom to replace @zero/@delete sugar with empty blocks.

Modified a few tests to make use of the new capability.