Implement Clone for @ and @mut types.

The borrowck-borrow-from-expr-block test had to be updated. I'm not sure why
it compiled before since ~int was already clonable.
Move tests inside clone.rs and fixed copyright headers.
auto merge of #5751 : metajack/rust/at-clones, r=thestinger

The borrowck-borrow-from-expr-block test had to be updated. I'm not sure why
it compiled before since ~int was already clonable.