Fewer Duplicate Villager Trades

At present, when a villager chooses a profession, or when a villager levels up, he chooses his new trades purely at random from those available for his profession.

This idea is for the newly selected trades to be biased towards those which have the fewest villagers in that village offering them.

For example, if a village's only armorer has a coal trade and an iron helmet trade, then the next villager to choose the armorer profession will be more likely to have iron chestplate trade, iron legging trade, and iron boot trade and less likely to have a coal trade or an iron helmet trade.

As a result of this change, it should be easier (but not trivial) for a player to acquire one librarian with each possible enchanted book trade, or one fletcher with each tipped arrow trade, or one farmer with each possible suspicious stew, etc.