feat: add get_disjoint_mut

[cmrschwarz]

Adds a get_disjoint_mut function similar to the ones on std::slice, std::collections::HashMap or even crates like indexmap. I think its fair to say that the ecosystem is standardizing on this mechanism.

I figured that there was no point in mirroring the more complicated API from [T]::get_disjoint_mut, as this crate cannot return slices of elemens anyways due to the Entry wrapper enum.

I therefore implemented the simpler version as precedented by hasmap and indexmap.

[cmrschwarz]

Sorry for missing the low msrv. I added a slab_no_const_generics cfg similar to the slab_no_const_vec_new we already have.
That should fix the problem.

[cmrschwarz]

Hmm, this is trickier than I thought.

It seems like on 1.31 we get an error despite the code being cfg'ed out,
presumably because the parser of that version does not understand const in a generic argument list.

I think I have a workaround by moving the implementation to a separate file which is not included on old rust versions.

I'm very sorry for the repeated CI runs, I now tested this using cargo hack locally so we should be good.

[cmrschwarz]

Hmm, #150 removed the build script that this relied on.

Since we are in the process of bumping MSRV anyways, would you consider bumping slightly further to MSRV 1.51 for const generics?

Similar reasoning applies to it, it was released over 4 years ago (March 25, 2021),
and is available in Debian stable (rustc on Debian bookworm is 1.63.0).
The top ten reverse dependencies all have higher MSRVs according to #150 (comment)

If this is acceptable I will gladly update this PR.

[Darksonn]

@paolobarbolini @taiki-e thoughts?

[taiki-e]

MSRV bump to 1.51 is okay.

[cmrschwarz]

Awesome. The PR is rebased now.

[cmrschwarz]

Any updates on this?

[Darksonn]

Thanks.