v3.3.0

• callr now sets the .Last.error variable for every uncaught callr
  error to the error condition, and also sets .Last.error.trace to its
  stack trace. If the error originates in the subprocess, then .Last.error
  is a hierarchical error object, and .Last.error.trace merges the
  traces from the two processes. See the README.md for an example.

• New $traceback() method for r_session, to run traceback() in the
  subprocess, after an error.

• A callr subprocess now does not load any R packages by default.

• New vignette, that showcases r_session.