Add a 'never' type

[dcodeIO]

There are situations where an error in a sub-expression can result in the type of something expected to have a type remaining void, like in a += 1 if a is not found, ultimately hitting an assertion. I added a naive workaround for this in #1046, but a never type indicating this condition would make more sense in that we can for example suppress follow-up errors further down the road.

[dcodeIO]

Closing this issue as part of 2020 vacuum because it seems unlikely to be picked up, and is rather an internal thing. It also isn't necessary to keep this open for me to remember, and there might be alternatives to a never type.