* Add field for universe level to TType
This doesn't do anything yet, other than introduce new universe
variables whenever we introduce a new type, but it's the first step
towards checking the universe hierarchy. Next step is to add constraints
when checking pi, unifying/converting types, and when adding data
constructors.
* TTC version increment
Thought I'd done this, but apparently I didn't save the file. Oops!
* Add structure for universe constraints
* Fix display of ambiguity errors
We need to store the Context in errors at the point where the error
occurs, or we might get some nonsense in the message. There's still a
couple of places in Error where we don't do this right. This fixes one
of them, and improves a few messages in the process.
