RFC: Remove move/copy keywords in favor of type-based solution

[nikomatsakis]

As discussed here:

http://smallcultfollowing.com/babysteps/blog/2012/10/01/moves-based-on-type/

If we decide not to take this route, then I think there is a separate bug to rationalize our rules for when a value is moved and when it is not.

[catamorphism]

This is implemented.