check impossible clauses

Also clean up some comments.

We now have types in constraints, but are still using values from
context.

tests/Possible.newt.fail

@@ -0,0 +1,10 @@
+*** Process tests/Possible.newt
+module Prelude
+module Possible
+ERROR at tests/Possible.newt:6:5--6:8: possible constructors: Prelude.Z, Prelude.S
+  
+  foo : Nat → Nat
+  foo ()
+      ^^^
+
+Compile failed