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())
}