Avoid accumulating slop in the pinned_object_block.
The pinned_object_block is where we allocate small pinned ByteArray# objects. At a GC the pinned_object_block was being treated like other large objects and promoted to the next step/generation, even if it was only partly full. Under some ByteString-heavy workloads this would accumulate on average 2k of slop per GC, and this memory is never released until the ByteArray# objects in the block are freed. So now, we keep allocating into the pinned_object_block until it is completely full, at which point it is handed over to the GC as before. The pinned_object_block might therefore contain objects which a large range of ages, but I don't think this is any worse than the situation before. We still have the fragmentation issue in general, but the new scheme can improve the memory overhead for some workloads dramatically.