Add block/meta and item/meta pair classes

[RecursivePineapple]

This PR adds a few util classes that I've needed several times in different mods. I've also seen similar classes replicated a good amount of times, so I figure these should be formalized properly.

If the mutability contract isn't clear, let me know. I tried to make it as robust as possible.

[github-actions]

Warning: 2 uncommitted changes
#99

[YannickMG]

Final nitpick : the contract description on the mutable versions kind of makes it sound like there's an immutable implementation class which you could use instead. Maybe clarify that you meant the exposed type and not the implementation used.