This function vendors cpp11 and cpp11eigen into your package by copying
the cpp11 and cpp11eigen headers into the `inst/include` folder and
adding 'cpp11 version: XYZ' and 'cpp11eigen version: XYZ' to the top of
the files, where XYZ is the version of cpp11 and cpp11eigen currently
installed on your machine.
Vendoring places the responsibility of updating the code on
you. Bugfixes and new features in cpp11 and cpp11eigen will not be
available for your code until you run `cpp_vendor()` again.