Add a "set_copyable" op to the collections extensions

[doug-q]

The existing set op unconditionally returns the old value in the list. This is necessary when the elements of the list have a linear type, but when the elements of the list have a copyable type this can be inefficient for large elements.

I suggest we rename the existing set to swap, and add set which does not return the old value and which requires that the element type is copyable.