Correctly handle inventory indexing

[caelunshun]

I noticed that the handling of the Creative Item packet doesn't take into account the "raw slot" logic:

https://github.com/caelunshun/feather/blob/d3d05d34eefe2047c374a66266e35715ebd9e2b1/server/src/player/inventory.rs#L184-L191

Essentially, packet.slot won't match the "inventory slot" properly. For example, "Creative Slot #0" > is not the hotbar, but the helmet slot. This mapping depends on the "top view" and "bottom view" of the inventory (think of the slots in a crafting bench, vs. the slots in a survival inventory with the equipment).

In Bukkit, there is this concept of "InventoryView" which has a convert-slot function to handle this.

This is an issue because there is a desync as to which item is currently held by the player: using the "hotbar index" for the inventory slot will give the wrong item.

Originally posted by @Momothereal in #46 (comment)

[caelunshun]

Yes, I see what you mean. Inventory slot indices are currently numbered as per https://wiki.vg/Inventory#Player_Inventory, which is only valid for the normal inventory—not for when e.g. the player is interacting with a crafting table (although those aren't implemented, so I haven't observed the problem).

I'm not sure of the ideal way to fix this in a Rust-like way. Perhaps we could use an enum type with a variant for each possible slot and then map those variants to raw slot IDs depending on the inventory type.