Can't export a type which refers to a private name. This has caught a
couple of visibility errors in the libraries, code and tests, so they've
been updated too.
This is mostly to make it easier to write linear function types without
having to invent names for everything, which might be noisy. Also it
improves the display of linear function types when the name isn't used
in the scope.
Still a couple of things to resolve in coverage and totality checking
before we can switch on %default, so don't expect quite the right
behaviour just yet. More progress though!
Also working on this has caught a few totality errors in the Idris 2
code base that Idris 1 missed... so these are fixed on the way.
