Prepare for indexmap 2.0

[cuviper]

Of the list in #135, the ones addressed here are:

• Increase MSRV to 1.56
• Always use an explicit feature for std
• Make get_index_mut return &K

I also added a few new features:

• Add MutableKeys::get_index_mut2
• impl Debug for IterMut and ValuesMut
• Add shrink_to

[cuviper]

Regarding other points from #135:

• Change the default hasher -- I think we should still consider this if we choose something no_std and hide it in a newtype. I don't think it's necessary though, especially now that we have a cleaner std story.
• Parameterize the index type -- that would be Parameterize the index type of maps and sets #147, and I would like to see that now, or else abandon the idea. It can be rebased and merged after this PR though.
• Maybe change the default remove()? I think we're ok as-is.
• Consider ordered PartialEq + Eq -- now is the time to talk about it, but I'm inclined to leave this alone. But we could merge Add dynamically-sized slices of maps and sets #177 which has that behavior in slices.

[bluss]

Awesome. Thanks a lot, and sorry for the long delay

[cuviper]

Alright, I've rebased, and I pushed an indexmap-1.x branch for older stable work. Let's merge and see about those other features separately!