Some loot refactoring

[Logg-y]

Internal changes:

• Loot selection (Generate/SelectTierItem) are gone, all this stuff is moved to inc_lootselect. This offers functions that can do more things than before, and hopefully offers functions with less confusing names that were there before. The internal mechanics are unfortunately more complicated due to accepting custom weightings for everything but most usage should be able to go through one of the exposed helpers instead.
• Loot assignment in parties moved to inc_lootowing
• Updated all the scripts that used the old functions to use the new equivalents
• Minor, but moved boss/semiboss adjustment to creature XP values inside the XP function, because all users of it were doing exactly the same logic externally on the return value anyway...
• Exposed a couple of operations of the personal loot system that might be useful elsewhere.

party_credit is still quite a long thing. It could be split up, but I'm not sure about that.

Actual functionality changes:

• Implemented "first boss kill drops an equippable item of tier based on ACR", aka that feature that made me want to clean this stuff up because making it under the old system would have just added to the pile of confusing functions.
• Expected loot value from containers turned out to be very slightly (think like 1%) different according to the newly rewritten debug stuff and the old. I'm not entirely sure why.

I have spent an hour or two running around locally (and fixed some silly mistakes), but in its current state it seemed okay.