Fix compilation error for get_mut_or_tagged_slot function

[kalabukdima]

Build in stable branch was failing with an error:

error[E0505]: cannot move out of `_` because it is borrowed
   --> src/slab.rs:460:13
    |
454 |     fn get_mut_or_tagged_slot(&mut self, id: Id<T>) -> Result<&mut T, Option<&mut TaggedSlot<T>>> {
    |                               - let's call the lifetime of this reference `'1`
...
457 |                 slot: Slot::Occupied { ref mut value },
    |                                        ------------- borrow of value occurs here
458 |                 tag,
459 |             }) if tag == id.tag => Ok(value),
    |                                    --------- returning this value requires that borrow lasts for `'1`
460 |             tagged_slot => Err(tagged_slot),
    |             ^^^^^^^^^^^ move out of value occurs here

Explanation of a similar problem can be found here.
This is the least messy fix that I could come up with.

Interestingly, there is no such problem with a get_or_tagged_slot function only because multiple non-mutable borrows of the same value are allowed.

BTW, should I bump the package version with this PR?

[cristicbz]

Thanks a lot for the PR, this looks like a regression-from-stable-to-stable to me! I suggested an alternative which seems to avoid the unreachable!, still pulling out the if-guard and occupied-check into separate matches. I think it's nicer---see what you think and let me know.

Also, yeah, it'd be great if you could also bump up the version number, so I can publish the new version, thanks!

[cristicbz]

Awesome, thanks! On the "double runtime check" didn't mean perf-wise, just meant style-wise