Make vectors and strings not implicitly copyable

[msullivan]

They are unique, and copying them is expensive. It shouldn't happen implicitly. However, we do it a lot, and getting rid of all of them is somewhat daunting...

[msullivan]

This is part of #2431.

[Dretch]

Would it be possible to somehow only copy them when sending them between tasks, and use reference-counting the rest of the time?

[msullivan]

The current plan is to make this configurable with a default of 'not implicitly copyable' and then override the default for all of our existing projects...