Data structures that cannot be moved (e.g., self-referential ones) should be properly tagged with Pin

[wedsonaf]

This gives a clear indication to the user of the data structure that they cannot be safely moved.

[alex]

What's the remaining scope for this now that we did chrdev?

[alex]

Per our call this weekend, I'm going to go ahead and close this. If we discover mroe places we ought to be using Pin, we can give them their own issues.