Position information for backend errors

Parse / typecheck errors have nice position location, and can be pretty printed, however the position information is not passed down the chain pass the typechecker. This means that errors in the SMT backend (e.g. [here](https://github.com/ethereum/act/blob/master/src/SMT.hs#L412)), cannot provide a nice pretty printed output showing the user the source of the issue.

I'm not sure what the best way to deal with this is, dragging around the position information in the refined AST seems overly clunky. @kjekac @MrChico do you have any suggestions?

Start with the SMT backend error path mentioned at src/SMT.hs#L412 and trace how position information is handled after parsing and typechecking. Review the discussion about carrying positions in the refined AST before deciding on an approach; done means backend errors can produce pretty-printed source locations.

Written by the indexing model from the issue text.