[aksiome]

Backports two improvements from Ward:

• Data assertion errors only show the part of the data the path reached, instead of the whole entity, block or storage data.
• New dummy commands: drop from <slot> [all], mainhand <slot> and use as a short form of use item.

Before:

Expected data at a.b.d, found {a:{b:{c:1},list:[1,2]}, ...}

After:

Expected data at a.b.d, found {c:1} at a.b

mainhand <slot> does the same as selectslot <slot>, but counts from 0 to 8. Ward started with only mainhand, because I find 1 to 9 misleading next to how Minecraft numbers slots everywhere else (hotbar.0 to hotbar.8). Ward later added selectslot for compatibility with packtest, and this PR adds mainhand here so both commands exist in both mods. I personally think it would be best to deprecate selectslot in favour of mainhand, since its numbering is closer to what people expect in commands. What do you think?