prevent destructuring from structs w/ destructors

[nikomatsakis]

This is more of a "preventive" bug than a current one. We have to be careful that if a struct has a destructor you cannot pull it apart using destructuring and unary move:

 let {f, g, h, _} = move foo

I am sure that our first version will have this bug, so I am filing it in advance. :)

[nikomatsakis]

Note: this came up in #2955