Add support for EUI-64 MAC addresses as macaddr8

This adds in support for EUI-64 MAC addresses by adding a new data type
called 'macaddr8' (using our usual convention of indicating the number
of bytes stored).

This was largely a copy-and-paste from the macaddr data type, with
appropriate adjustments for having 8 bytes instead of 6 and adding
support for converting a provided EUI-48 (6 byte format) to the EUI-64
format.  Conversion from EUI-48 to EUI-64 inserts FFFE as the 4th and
5th bytes but does not perform the IPv6 modified EUI-64 action of
flipping the 7th bit, but we add a function to perform that specific
action for the user as it may be commonly done by users who wish to
calculate their IPv6 address based on their network prefix and 48-bit
MAC address.

Author: Haribabu Kommi, with a good bit of rework of macaddr8_in by me.
Reviewed by: Vitaly Burovoy, Kuntal Ghosh

Discussion: https://postgr.es/m/CAJrrPGcUi8ZH+KkK+=TctNQ+EfkeCEHtMU_yo1mvX8hsk_ghNQ@mail.gmail.com