Reduce the minimum allocable chunk size to 8 bytes (from 16). Now that

Reduce the minimum allocable chunk size to 8 bytes (from 16).  Now that
ListCells are only 8 bytes instead of 12 (on 4-byte-pointer machines
anyway), it's worth maintaining a separate freelist for 8-byte objects.
Remembering that alloc chunks carry 8 bytes of overhead, this should
reduce the net storage requirement for a long List by about a third.