[HACKERS][REVIEW] macaddr 64 bit (EUI-64) datatype support

Hello,

I've reviewed the patch[1].

Result of testing:

make installcheck-world:  tested, passed
Implements feature:       tested, passed
Spec compliant:           tested, passed
Documentation:            tested, passed


The patch introduce a new type macaddr8 for EUI-64 addresses[2]
(assuming OUI field is 24 bits wide) with EUI-48 (existing "macaddr"
type) interoperability.
It is a mostly copy-pasted macaddr implementation with necessary
changes for increased range.
Consensus was reached on such implementation in the current thread before.

There are two patch files for convenient reviewing: base macaddr8
implementation and its supporting in btree-gin and btree-gist indexes.

The patch:
* cleanly applies to the current master
(6af8b89adba16f97bee0d3b01256861e10d0e4f1);
* passes tests;
* looks fine, follows the PostgreSQL style guide;
* has documentation changes;
* has tests.

All notes and requirements were took into account and the patch was
changed according to them.
I have no suggestions on improving it.

The new status of this patch is: Ready for Committer

P.S.:
1. The doc and error/hint messages should be proof-read by a native speaker.
2. A committer should bump catversion. It is not bumped in the patch
because it is unclear when it is committed.

[1]https://postgr.es/m/CAJrrPGeT8zrGPMcRVk_wRvYD-ETcgUz6WRrc2C=iNubMRkrMxw@mail.gmail.com
[2]http://standards.ieee.org/develop/regauth/tut/eui64.pdf
-- 
Best regards,
Vitaly Burovoy