In-place-dropping setters for `UnmanagedSignalCell`

[Tamschi]

Is your feature request related to a problem? Please describe.
I'd like to have additional setters that are guaranteed to drop the previous value in place, both unconditional variants (like replace) and ones that use PartialEq (like change). Returning the previous value makes unconditional …_eager Futures larger than they'd otherwise need to be.

Describe the solution you'd like
The implementation can be just about the same as that of the replace and change methods, but without returning the value.
(The current implementation of the deferred versions of these accessors in fact already does that, but doesn't guarantee it.)

Describe alternatives you've considered
-

Additional context
I forgot this somewhere along the way 😬
Most likely when I decided that I didn't want an accessor called set for cells, as I want to encourage use of change instead.

Since this is a breaking change, it should be developed against the flourish-next branch.

[Tamschi]

I'm still thinking on which names to use here. In the meantime, you can guarantee a drop-in-place by calling the update methods with a closure that does so.