Gracefully ignore bad comprehensions instead of failing with an error.

Nested tuple flattening is now done all-at-once, like object domain flattening.
Add timing to main.py.
Fixed failing transformation test.
Fixed infinite loop when skipping bad aggregate query. Tolerate flattening an unused relation.
Added StmtTransformer.

This is a visitor for inserting statements immediately before or after the currently visited node.
Typo in update-rewriting rule.
Added 'assign_update' set operation.
Bugfix StmtTransformer.
Added rewriting for using runtimelib types. Tests broken.
Bugfix relation flattening.
Update tests for runtimelib type rewriting.
Silenced some verbose things.
Merge branch 'develop'