Use 128-bit math to accelerate some aggregation functions.

On platforms where we support 128bit integers, use them to implement
faster transition functions for sum(int8), avg(int8),
var_*(int2/int4),stdev_*(int2/int4). Where not supported continue to use
numeric as a transition type.

In some synthetic benchmarks this has been shown to provide significant
speedups.

Bumps catversion.

Discussion: 544BB5F1.50709@proxel.se
Author: Andreas Karlsson
Reviewed-By: Peter Geoghegan, Petr Jelinek, Andres Freund,
    Oskari Saarenmaa, David Rowley