Improve QFutureInterface/QPromise constructors

Addressing the latest (post-merge) commit feedback from Marc Mutz on QPromise [1]:

• Simplify QPromise move constructor/assignment (should improve construction performance in theory):

• Introduce move semantics to QFutureInterface
• Make construction noexcept (many STL containers use std::move_if_noexcept, so it might prove useful)

• Make QPromise::swap public and introduce qSwap/std::swap overloads for QPromise (might be beneficial)

Extra:

• Unify cancel and finish in QPromise destructor (now 2 separate calls to QFutureInterface::cancel() and QFutureInterface::reportFinished() -> should be a single call)

Note: some optimizations might also be useful for the QFuture (e.g. move semantics, swaps).

[1] https://codereview.qt-project.org/c/qt/qtbase/+/300337