propagate constraints during copying

[graydon]

Copying from one slot to another should add a copy of every constraint that
includes the RHS of the assignment to the copy postcondition (with the RHS
renamed, in the constraint, to the new LHS slot).

In other words:

let int x = 10;
let int y = 11;
check lt(x,y); // postcond = { lt(x,y) }
let int z = x; // postcond = { lt(x,y), lt(z,y) }

Unless some horrible complications arise that make this impossible, I believe
the current rules permit this propagation to work.

[graydon]

Shifted to rustc

[catamorphism]

Fixed in 182c413