Change barrel registration algorithm and remove reduntant code

[Thorinwasher]

Fixes #113

Should be a performance improvement, does take up a bit more memory, but this is probably miniscule compared to what every item in the barrel inventories takes.

As this is quite a major refactor (whops), I might have messed up somewhere.

Here's a list of all the things I did, but probably best to look at the code:

• Use HashMap when mapping barrels instead of using lists, much easier access when coding, and probably faster at Barrel amounts larger than 100
• Remove sign offsett thing completely, this property seems reduntant (edit: removes access through large barrels through the sign, up to discussion of this is okay)
• Listen to world load / unload events, and register / unregister barrels as needed. (untested and I'm not quite sure how this works)
• Does not validate barrels randomly anymore, I think with some other improvements on listeners, this can be mitigated

[Jsinco]

will fully review later today 👍