Include valgrind as part of CI

[saem]

In the past while it has become evident that there are a number of fundamnetal issues with our memory management implementations (refc, arc, and orc) that lead them to be unsafe, especially in conjunction with threading. Before even getting to complex multi-threading scenarios we can eliminate lots of issues by addressing valgrind's findings.

TODO:

• [ ] Enable valgrind for tests in CI; a number of tests already support this
• [ ] tackle a few quick wins to get some momentum