[ refactor ] Removed unused last argument from SplitError.CantSplit.
[ refactor ] Use whenJust and <.>.
[ comment ] Agda.Utils.NubList is UNUSED.
[ new ] Agda.Utils.Singleton replaces Agda.Utils.Pointed.

Overloaded method `singleton` constructs singleton collections of
various types.  Analogous to Agda.Utils.Null.
[ Issue 1084 ] Fixed by removing function names that do not appear in offending calls.

Termination errors should now only be reported for functions that
actually contribute to potential cycles.
[ Issue 1084 ] Updated error messages in test/fail.
[ Issue 1084 ] Test case.

Already covered by test cases for 921 and 1375, but it is clearer to
have a native test case.
[ refactor ] Null, Singleton instances for Favorites, CMSet, CallGraph.
[ comment ] Attempt to add callSource to CallPath (abandoned).