new - notably for checking UNexported utilities
- checkConv(.) now works with one *nested* list;
  one argument instead of two
- .makeCC() utility for constructing these
This reverts commit 96ec99d10e4a25af7e4c5dfeebaad9a9e9767f5f.
testthat -- new version:  expect_warning(<expr>, ....)
now fails when there is an error after the warning