Added get_mut_unchecked

[dynos01]

I added get_mut_unchecked() as it might help someone.

Also, there are two things I'm considering but currently unsure about how to correctly implement:

• Add wait_mut(). However mutable reference across threads requires Mutex, but waiting with Mutex acquired will block other threads from initializing the value. Is there a case that this will make sense?
• Add get_mut_unchecked() in imp_cs.rs. This seems to require modification of the upstream crate critical_section, so I skipped it for now.

[dynos01]

For point 2, this does compile, but will it work as expected? (I don't have experience with crate critical-section, and the definitions of borrow and borrow_mut here look different)

pub(crate) unsafe fn get_mut_unchecked(&mut self) -> &mut T {
    debug_assert!(self.is_initialized());
    // SAFETY: The caller ensures that the value is initialized and access synchronized.
    crate::unwrap_unchecked(self.value.borrow_mut().get_mut().get_mut())
}

[matklad]

What's the concrete use-case for this function? Given & -> &mut signature, this seems more or less impossible to use correctly.

[dynos01]

My initial purpose was to implement wait_mut, but I soon realized that wait_mut seemed meaningless. But yes, it should be &mut -> &mut here. Since there are get_unchecked and get_mut, will a get_mut_unchecked look reasonable?

[matklad]

I think that’s already available as get_mut().unwrap_unchecked()

[dynos01]

Then what about a shortcut that's self-explanatory? I've added a new commit to fix the function signature, but if you believe this is not necessary, feel free to close this PR. Thanks.