In preparation for Lilliput, I'd like to simplify BinList handling a bit.

BinList are a data structure that stores small blocks that had been prematurely deallocated for re-use. Due to the way BinList is implemented we need the blocks to be at least two words. That minimal block size causes ripples all across metaspace, including testing code. We can get rid of this complexity by shrinking a Block to just one word.