Validate transaction slots

[ShockedPlot7560]

Introduction

At present, we can place an item in any slot in an inventory (except in exceptional cases).

It is now necessary to provide a system for verifying transactions in order to guarantee a stable internal state for each inventory.

To maintain compatibility and avoid changing the Inventory interface. A new interface has been created, introducing a new notion of : "slot safe".
I named an inventory slot safe when we are guaranteed that the internal state is consistent with what it should do. So it's up to the core (for inventory implemented by pmmp) and plugin developers to make sure they implement the right validation logic.

Relevant issues

• Fixes A system to let transactions decide what stuff can go into a particular inventory slot #6262

Changes

API changes

• Add SlotSafeInventory
• Add SlotValidator

Behavioural changes

• ArmorInventory checked now correctly their slots.

Backwards compatibility

Enforced by the SlotSafeInventory

Follow-up

As proof of concept, I only implement the ArmorInventory logics. There is many inventory which need to check their slot.

[dktapps]

This could also be done with closures

[dktapps]

Concept looks fine.

However, as previously discussed on Discord, I think that all validators should accept the item in order to allow the transaction to proceed, as opposed to the current behaviour of only one. The current behaviour does not make sense to me - I think it's more likely that people will want to add extra restrictions instead of overriding existing ones

[ShockedPlot7560]

Alright, I think we are done with this.

[dktapps]

Looks good overall, minus a couple of nits and suggestions