MetaDataRef not fully functional

[S-S-X]

Implement rest of MetaDataRef:
https://github.com/mt-mods/mineunit/blob/master/metadata.lua

Basically simply just replace error("NOT IMPLEMENTED") with correctly arranged stream of bytes that will make it do something similar what comments above function signature says.

edit. well no comments at all in that file... sorry about that but here's what should fix it: https://minetest.gitlab.io/minetest/class-reference/#metadataref

[S-S-X]

NodeMetaRef and InvRef added, also few more methods implemented.

[S-S-X]

PR #14 implements many missing features but still not complete.

[S-S-X]

to_table for MetaDataRef was completely wrong, looked at engine code and hopefully fixed now ^

[S-S-X]

To verify NodeMetaRef:to_table() compatibility here's simple dump for that done with engine:

{
	fields = {
		tube_time = "0",
		formspec = "size[8,9;]list[context;src;3,1;1,1;]label[0,0;MV Tool Workshop]list[context;upgrade1;1,3;1,1;]list[context;upgrade2;2,3;1,1;]label[1,4;Upgrade Slots]list[current_player;main;0,5;8,4;]listring[current_player;main]listring[context;src]listring[current_player;main]listring[context;upgrade1]listring[current_player;main]listring[context;upgrade2]listring[current_player;main]",
		infotext = "MV Tool Workshop Idle",
		MV_EU_demand = "0",
		MV_EU_input = "0"
	},
	inventory = {
		upgrade2 = {
			ItemStack("")
		},
		upgrade1 = {
			ItemStack("")
		},
		src = {
			ItemStack("technic:lava_can")
		}
	}
}

I think it is correct already but there might still be -- TODO: comment about verifying it.

[S-S-X]

Removing InvList type completely, even for internal use.
InvRef will take over and handles all inventory list operation directly.

Some tests should be also added for verified correct behavior.