Give inet/cidr datatypes their own hash function that ignores the inet vs

Give inet/cidr datatypes their own hash function that ignores the inet vs
cidr type bit, the same as network_eq does.  This is needed for hash joins
and hash aggregation to work correctly on these types.  Per bug report
from Michael Fuhr, 2004-04-13.
Also, improve hash function for int8 as suggested by Greg Stark.