Support IPv4-mapped addresses

[urusha]

It would be nice if we could build IPv6 database with all IPv4 addresses included as IPv4-mapped IPv6 addresses (::ffff:*:*)
Currently, It could be done by editing ipv6 csv inside zip (country example):

tail -n+2 GeoIP2-Country-Blocks-IPv4.csv |
    awk -F, '{ split($1,a,"/"); split(a[1],a1,"."); m = 96+a[2]; printf("::ffff:%02x%02x:%02x%02x/%d,%s,%s,%s,%s,%s\n"),a1[1],a1[2],a1[3],a1[4],m,$2,$3,$4,$5,$6}' >> GeoIP2-Country-Blocks-IPv6.csv

[sherpya]

this may be an idea for #19

[foutrelis]

This would allow for GeoIPv6.dat to be used with ngx_http_geoip:

When using the databases with IPv6 support (1.3.12, 1.2.7), IPv4 addresses are looked up as IPv4-mapped IPv6 addresses.

Based on our downstream report in Arch, it used to be possible to lookup ::ffff:195.66.1.4 using one of the last GeoIPv6.dat databases published by MaxMind in April 2018.