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.