- Builtin.PrimOps: ReturnsAlg was used only for unboxed tuples.
  Rename to ReturnsTuple.
- Builtin.Utils: use SDoc for a panic message.
  The comment about <<details unavailable>> was obsoleted by e8d35677.
- TagCheck: fix wrong logic. It was zipping a list 'args' with its
  version 'args_cmm' after filtering.
- Core.Type: remove an outdated 1999 comment about unlifted polymorphic types
- hadrian: remove leftover debugging print