Consider T&& assignment operator

I think this would be desirable for switching out the type stored in a protocol. I think it's unlikely to result in any unexpected conversions, but it is introducing a bit of implict-ness.

```c++
protocol p(SomeType{});
// ...
p = OtherType{}; // ERROR: doesnt work right now
p = protocol(OtherType{});
``

Start by locating the protocol assignment and construction entry points, then inspect any existing assignment-related tests. Confirm that assigning OtherType{} directly to a protocol holding SomeType{} is accepted, while preserving the existing explicit construction behavior and conversion rules.

Written by the indexing model from the issue text.