Adding slot field to BlockPos, for future use with compound blocks

[dmchurch]

This adds a new integer property Slot to the BlockPos class, with the following behavior:

• Defaults to -1, defined as the constant BlockPos.SlotDontCare
• pos.Equals(other) now checks that Slot properties match
• pos.GetHashCode() includes the Slot value in the hash
• Any mutator method (Set, West, etc) will reset the Slot property to SlotDontCare
• Directly changing any of the BlockPos position fields from external code (by any value from -15..15) will cause the Slot property to self-reset to SlotDontCare
• Copy and CopyAndCorrectDimension will copy the Slot value
• The indexer signature now treats index 4 as the Slot property
• Two new slot-aware constructor signatures: BlockPos(int x, int y, int z, int dim, int slot) and BlockPos(Vec3i vec, int dim, int slot)
• One new mutator: public BlockPos SetSlot(int slot)
• Protobuf serialization serializes the Slot property as tag 4, but only if it is not currently set to the default SlotDontCare
• Byte-serialization (ToBytes() and CreateFromBytes()) do not preserve the slot value (only x/y/z/dimension) as those methods are only used for particles, which don't need to know about slots
• ToString() includes the slot value in the output if the value is not SlotDontCare

This should have no impact on the game at present, as there isn't any code that reads or writes the Slot property; all BlockPos instances will grow by 4 bytes in size, but that seems incredibly unlikely to change anything.