(PUP-2831) Make Puppet work with RGen 0.7.0

This updates the Gemfile to use RGen 0.7.0 (optionally).
In RGen 0.7.0 containment now works as specified, if something is
contained more than once, it moves to the last container that contains
it (by removing it as contained from all other containers). This caused
problems in the Function API where the block default Callable type was
reused for all signatures (and thus moved). There were also problems in
tests where the same type instances was contained in multiple locations.

This fixes these to use distinct instances.
(maint) Remove dead code
Merge pull request #2802 from hlindberg/PUP-2831_use-rgen-0-7-0

(PUP-2831) Make Puppet work with RGen 0.7.0