Implement "no implicit copies" scheme

[msullivan]

Something like the scheme proposed in http://smallcultfollowing.com/babysteps/rust/no-implicit-copies/ should be implemented.

[nikomatsakis]

I am now of the belief that the only implicit copies which ought to be illegal are:

• those copying mutable data
• those requiring a malloc (i.e., those containing unique pointers, vectors)

Implicit copies occur in various places, but one of the most common (and easiest to prevent/detect) is when closing over data in closures. We could easily add a check there as a start, but we should eventually capture all such cases.

[msullivan]

I'll be tackling this in phases, so I have opened some sub-bugs for tracking.

[msullivan]

I don't think there is anything left. Closing.