Sort support for macaddr8

Peter and I implemented this small (attached) patch to extend
abbreviated key compare sort to macaddr8 datatype (currently supported
for macaddr).

I tried checking to see if there is a performance difference using the
attached DDL based on src/test/regress/sql/macaddr8.sql. I found
that the sort function is only exercised when creating an index (not,
for example, when doing some type of aggregation).

With the patch applied to current master and using the DDL attached,
the timing for creating the index hovered around 20 ms for master and
15 ms for the patched version.

Machine and version specs: PostgreSQL 12beta1 on x86_64-pc-linux-gnu
compiled by gcc (Ubuntu 8.3.0-6ubuntu1) 8.3.0, 64-bit

I think that that seems like an improvement. I was thinking of
registering this patch for the next commitfest. Is that okay?

I was just wondering what the accepted way to test and share
performance numbers is for a small patch like this. Is sharing DDL
enough? Do I need to use pg_bench?

--
Melanie Plageman