(PUP-2914) Add rgen 0.7.0 to vendored libraries

This commit adds the rgen 0.7.0 library by placing it in the vendor lib
that is needed for development on the new parser.
(PUP-2914) Remove rgen 0.7.0 dependency

The library rgen was an unnecessary dependency since it was
vendored. This commit removes unnecessary dependency.
(doc) Add Puppet Specific Documentation

Vendored libraries had no way for us to tell where they had come from or
what version the library was at.
This commit adds `PUPPET_README.md` files to each of our vendored
libraries to give us more information about where they came from.
Merge remote-tracking branch 'upstream/pr/2995'

* upstream/pr/2995:
  (doc) Add Puppet Specific Documentation
  (PUP-2914) Remove rgen 0.7.0 dependency
  (PUP-2914) Add rgen 0.7.0 to vendored libraries