This function vendors cpp11 and cpp11armadillo into your package by copying
the cpp11 and cpp11armadillo headers into the `inst/include` folder and
adding 'cpp11 version: XYZ' and 'cpp11armadillo version: XYZ' to the top of
the files, where XYZ is the version of cpp11 and cpp11armadillo currently
installed on your machine.
Vendoring places the responsibility of updating the code on
you. Bugfixes and new features in cpp11 and cpp11armadillo will not be
available for your code until you run `cpp_vendor()` again.