Change uint128 from [2]uint64 to a struct of hi/lo to appease compiler

[bradfitz]

If golang/go#24416 gets fixed we could revert this.

name                              old time/op    new time/op    delta
StdIPv4-4                            361ns ± 9%     386ns ± 8%     ~     (p=0.167 n=5+5)
IPv4-4                               273ns ±12%     301ns ±14%     ~     (p=0.238 n=5+5)
IPv4_inline-4                        251ns ± 5%     262ns ±10%     ~     (p=0.730 n=5+5)
StdIPv6-4                            507ns ±18%     490ns ±13%     ~     (p=0.651 n=5+5)
IPv6-4                               414ns ± 8%     416ns ± 6%     ~     (p=1.000 n=5+5)
IPv4Contains-4                      30.1ns ± 4%    13.1ns ±12%  -56.41%  (p=0.008 n=5+5)
ParseIP/v4-4                        55.9ns ± 5%    49.5ns ±10%  -11.35%  (p=0.032 n=5+5)
ParseIP/v6-4                         244ns ± 9%     217ns ± 8%  -10.76%  (p=0.040 n=5+5)
ParseIP/v6_ellipsis-4                157ns ±12%     157ns ± 7%     ~     (p=0.730 n=5+5)
ParseIP/v6_v4-4                      191ns ±12%     168ns ±19%     ~     (p=0.310 n=5+5)
ParseIP/v6_zone-4                    464ns ±21%     448ns ±12%     ~     (p=0.841 n=5+5)
StdParseIP/v4-4                      186ns ± 5%     186ns ±13%     ~     (p=1.000 n=5+5)
StdParseIP/v6-4                      362ns ± 7%     330ns ± 5%   -8.68%  (p=0.016 n=5+5)
StdParseIP/v6_ellipsis-4             258ns ±17%     280ns ± 6%     ~     (p=0.254 n=5+5)
StdParseIP/v6_v4-4                   427ns ±28%     457ns ±15%     ~     (p=0.548 n=5+5)
StdParseIP/v6_zone-4                 285ns ±17%     305ns ±13%     ~     (p=0.421 n=5+5)
IPString/v4-4                        153ns ±21%     122ns ±17%  -19.79%  (p=0.048 n=5+5)
IPString/v6-4                        356ns ± 3%     345ns ±22%     ~     (p=0.151 n=5+5)
IPString/v6_ellipsis-4               346ns ±13%     329ns ±20%     ~     (p=0.548 n=5+5)
IPString/v6_v4-4                     309ns ±15%     266ns ±11%  -13.90%  (p=0.032 n=5+5)
IPString/v6_zone-4                   362ns ±18%     330ns ±21%     ~     (p=0.421 n=5+5)
IPPrefixMasking/IPv4_/32-4          54.8ns ± 4%    15.1ns ± 4%  -72.44%  (p=0.008 n=5+5)
IPPrefixMasking/IPv4_/17-4          54.9ns ± 4%    14.6ns ± 5%  -73.38%  (p=0.008 n=5+5)
IPPrefixMasking/IPv4_/0-4           55.0ns ± 4%    13.5ns ± 6%  -75.50%  (p=0.008 n=5+5)
IPPrefixMasking/IPv6_/128-4         55.0ns ± 1%    15.5ns ± 7%  -71.79%  (p=0.008 n=5+5)
IPPrefixMasking/IPv6_/65-4          54.2ns ± 2%    14.6ns ± 5%  -73.04%  (p=0.008 n=5+5)
IPPrefixMasking/IPv6_/0-4           55.4ns ± 2%    14.9ns ± 9%  -73.06%  (p=0.008 n=5+5)
IPPrefixMasking/IPv6_zone_/128-4    54.4ns ± 1%    15.0ns ±13%  -72.44%  (p=0.008 n=5+5)
IPPrefixMasking/IPv6_zone_/65-4     55.5ns ± 2%    14.8ns ±16%  -73.31%  (p=0.008 n=5+5)
IPPrefixMasking/IPv6_zone_/0-4      54.8ns ± 4%    14.7ns ± 8%  -73.23%  (p=0.008 n=5+5)
ParseIPPort/v4-4                     118ns ± 8%     103ns ± 9%  -12.88%  (p=0.008 n=5+5)
ParseIPPort/v6-4                     318ns ±12%     262ns ± 6%  -17.38%  (p=0.008 n=5+5)
ParseIPPort/v6_ellipsis-4            243ns ±10%     240ns ±19%     ~     (p=0.841 n=5+5)
ParseIPPort/v6_v4-4                  279ns ±14%     237ns ±16%  -14.91%  (p=0.040 n=5+5)
ParseIPPort/v6_zone-4                578ns ± 9%     546ns ± 7%     ~     (p=0.222 n=5+5)
IPRangePrefix-4                      142ns ± 5%      61ns ±11%  -56.74%  (p=0.008 n=5+5)
IPSetFuzz-4                         41.0µs ±16%    33.3µs ±15%  -18.87%  (p=0.016 n=5+5)

Fixes #102

[danderson]

Merging this so I can work on cleaning up some code that depends on the representation of the uint128.