Allow returning non-copyable types in with-style methods

[czechboy0]

As we're getting close to a 1.0, if this is needed, we should change it now. @FranzBusch?

[czechboy0]

Actually, a broader question - how can we avoid any needless limits on the return value in any with-style functions (like watch)?

What about:

func withFoo<Return: ~Copyable>(_ body: (Foo) async throws -> sending Return) async throws -> sending Return

This allows the user to create a non-copyable value and return it from the closure, whereas previously they couldn't (for no good reason). And by adding sending, users can also return a non-sendable value.

[FranzBusch]

With nonisolated(nonsending) the return value doesn't have to be sending. Just a regular return is fine. Though allowing ~Copyable values are good.