v3.4.0

• All callr functions and background processes properly clean up
  temporary files now (#104).

• callr now uses a more principled setup for the library path, and
  restores the related environment variables in the child process.
  This is a breaking change if you relied on having the library set
  in a system() subprocess of the callr subprocess (#114).

• Better printing of rlang_errors that happened in the subprocess.

• The stacking of error objects is slightly different now, as we keep the
  unmodified error from the subprocess in $parent$error.

• callr now loads .Rprofile files from the current working directory
  by default. This works better with packrat, renv, and other software
  that relies on a local profile for initialization (#131).