ropensci/tarchetypes: More resilient automatic names for tar_map() targets

tarchetypes 0.3.0 Invalidating changes

• When names = NULL in tar_map(), use hashes instead of numeric indexes for generated target names (#67). That way, target names are no longer sensitive to the order of values, and so targets will incorrectly invalidate less often. Unfortunately, this is an invalidating change: some targets will automatically rerun after you install this version of tarchetypes. I apologize for the inconvenience this causes. However, we do need this patch in order to solve #67, and targets will incorrectly invalidate less frequently in the future.

• Migrate to utilities for error handling and metaprogramming exported from targets (#59).