future: optimize `.then()` for unique rvalues.

[griwes]

.then() chains are currently suboptimal. Consider:

auto fut = make_ready_future()
    .then(some_callback)
    .then(other_callback)
    .then(yet_another_callback);

Only the result of the final .then() needs to be truly stateful - the temporary ones used to construct this continuation chain are never going to be used except to feed the singular continuation. There's space for optimization here.