RAW_LONG_SLOTS_

Avail integers should always occupy the fewest number of slots to unambiguously indicate the represented integer. A zero integer is represented by a single slot containing a zero Int. Any Int can be converted to an Avail integer by using a single slot, and any long can be represented with at most two slots.

Thus, if the top slot is zero (0), the second-from-top slot must have its upper bit set (fall in the range -0x80000000..-1), otherwise the last slot would be redundant. Likewise, if the top slot is minus one (-1), the second-from-top slot must have its upper bit clear (fall in the range 0..0x7FFFFFFF).