General cleanup of IdDict classes, with tests

Extend existing tests, introduce new tests. Apply 'rule of zero' where possible and turn IdDictLabel into a simple struct. Make IdDictField do bounds checking when retrieving by index, and activate the appropriate test. Default some methods where possible (use baseclass default) and make 'do nothing' methods defined in the header file.

Test of IdDictRange, in particular the main 'build_range' method. More tests and refactoring will come in future MRs, this seems an appropriate point to pause.