plrust should clean up artifacts

[BradyBonnette]

In a conversation with @eeeebbbbrrrr in talking about #81 we discovered that there are some opportunities to have plrust clean up artifacts at various points

• Adding callbacks on transaction abort and pre-commit to recursively delete crate directory
• Adding callbacks for drop schema and drop function
• Discuss target dir (e.g. shared compiled dependencies <root level scratch directory>/target)
• Discuss using database directory for shared library storage (e.g. postgres' data_directory GUC) so that any built artifacts can also be dropped in the event of a drop database call

[eeeebbbbrrrr]

We no longer leave anything on disk (outside of the target/ directory) after a successful compile.