feat: implement InPlaceWrite<T> for &'static mut MaybeUninit<T>

[Ddystopia]

Simple PR to implemente InPlaceWrite for a mutable reference. In my code the use case is to use StaticCell::uninit() to get &'static mut MaybeUninit<T>.

[BennoLossin]

Oh yeah that works, thanks for the contribution by the way!

Can you add a commit message containing a description of your change? Also the title of the commit contains feat (& a typo), I don't use that convention. Have you read the contribution guidelines they contain some useful information and links?

[Ddystopia]

I updated the commit message, is it fine? For the safety comment, I borrowed it from Pin::static_mut documentation.

That CI failure, it seems flaky. Like, no code had changed and tests pass on my machine.

[BennoLossin]

I think it would be much better to use Pin::static_mut and MaybeUninit::assume_init_mut to obtain the pinned reference. Then you only need the justification for the initialized memory.
(same for write_init I guess? (without Pin of course))

[BennoLossin]

I updated the commit message, is it fine? For the safety comment, I borrowed it from Pin::static_mut documentation.

That CI failure, it seems flaky. Like, no code had changed and tests pass on my machine.

Yeah the ui tests also failed this morning on the main branch, the nightly output has changed, I'll wait a few days to see if that's intentional.

Can you also add a bullet point about this change into the changelog?

The commit message needs a bit more motivation, would be great if you could mention that you want to use it with static_cell. Also I wouldn't say that this "avoids allocations", but rather enables users to use external allocation mechanisms (like static_cell).