implement copy operation

We should have a `copy` expression, applicable to a place. Writing `x.copy` has no impact if `x` is a `copy` type (shared, our) and it is illegal if `x` is `leased`. However, for a `my` place it (a) produces an `our` value and (b) changes the type of the copied place `x` from `my Foo` to `our Foo`. 