Change "short" data items from 127B or less to be 63B or less, and use the
bit free'd up to mark short keys with a prefix-compression byte.   Ref #600.
When we aren't able to create a short key or value (and, in the case of a
value, there's no associated RLE), the key or value is at least 64B, else
we'd have been able to store it as a short cell.  Decrement/Increment the
size before storing it, in the hopes that relatively small key/value sizes
will pack into a single byte instead of two bytes.  Ref #600.
Add a comment to explain why we don't need to check the new value's
length, in the case of a short value cell, during dictionary lookup.