* removed dupl lint errors in d2compression/huffman.go
* remove duplicate code for set/unique item properties
This is a more involved edit. I've added a `PropertyDescriptor`
which is a common struct that should have existed already. The
`PropertyDescriptor` is used to generate property instances, and is common
to item affixes, set items, sets, unique items, and runewords.
This was all to remove duplicate code in d2item/
* removed duplicate code for rare item affixes
