Change the representation of IP to a pair of uint64s.

This allows IP manipulation operations to operate on IPs as two registers, which empirically
leads to significant speedups, in particular on OOO superscalars where the two halves can
be processed in parallel.

You might expect that we could keep the representation as [16]byte, and do a cycle of
BigEndian.Uint64+tweak+BigEndian.PutUint64, and that would compile down to efficient
code. Unfortunately, due to a variety of missing optimizations in the Go compiler,
that is not the case and that code turns into byte-wise operations. On the other hand,
converting to a uint64 pair at construction results in efficient construction (a pair of
MOVQ+BSWAPQ) and efficient twiddling operations (single-cycle arithmetic on 64-bit
pairs). See also:
 - golang/go#41663
 - golang/go#41684
 - golang/go#42958
 - The discussion in inetaf#63

name                              old time/op    new time/op    delta
StdIPv4-8                            146ns ± 2%     141ns ± 2%    -3.42%  (p=0.016 n=5+5)
IPv4-8                               120ns ± 1%     107ns ± 2%   -10.65%  (p=0.008 n=5+5)
IPv4_inline-8                        120ns ± 0%     118ns ± 1%    -1.67%  (p=0.016 n=4+5)
StdIPv6-8                            211ns ± 2%     215ns ± 1%    +2.18%  (p=0.008 n=5+5)
IPv6-8                               281ns ± 1%     252ns ± 1%   -10.19%  (p=0.008 n=5+5)
IPv4Contains-8                      11.8ns ± 4%     4.7ns ± 2%   -60.00%  (p=0.008 n=5+5)
ParseIPv4-8                         68.1ns ± 4%    78.8ns ± 1%   +15.74%  (p=0.008 n=5+5)
ParseIPv6-8                          419ns ± 1%     409ns ± 0%    -2.40%  (p=0.016 n=4+5)
StdParseIPv4-8                      73.7ns ± 1%    88.8ns ± 2%   +20.50%  (p=0.008 n=5+5)
StdParseIPv6-8                       132ns ± 2%     134ns ± 1%      ~     (p=0.079 n=5+5)
IPPrefixMasking/IPv4_/32-8          36.3ns ± 3%     4.8ns ± 4%   -86.72%  (p=0.008 n=5+5)
IPPrefixMasking/IPv4_/17-8          39.0ns ± 0%     4.8ns ± 3%   -87.78%  (p=0.008 n=5+5)
IPPrefixMasking/IPv4_/0-8           36.9ns ± 2%     4.8ns ± 4%   -87.07%  (p=0.008 n=5+5)
IPPrefixMasking/IPv6_/128-8         32.7ns ± 1%     4.7ns ± 2%   -85.47%  (p=0.008 n=5+5)
IPPrefixMasking/IPv6_/65-8          39.8ns ± 1%     4.7ns ± 1%   -88.13%  (p=0.008 n=5+5)
IPPrefixMasking/IPv6_/0-8           40.7ns ± 1%     4.7ns ± 2%   -88.41%  (p=0.008 n=5+5)
IPPrefixMasking/IPv6_zone_/128-8     136ns ± 3%       5ns ± 2%   -96.53%  (p=0.008 n=5+5)
IPPrefixMasking/IPv6_zone_/65-8      142ns ± 2%       5ns ± 1%   -96.65%  (p=0.008 n=5+5)
IPPrefixMasking/IPv6_zone_/0-8       143ns ± 2%       5ns ± 3%   -96.67%  (p=0.008 n=5+5)
IPSetFuzz-8                         22.7µs ± 2%    16.4µs ± 2%   -27.84%  (p=0.008 n=5+5)

name                              old alloc/op   new alloc/op   delta
StdIPv4-8                            16.0B ± 0%     16.0B ± 0%      ~     (all equal)
IPv4-8                               0.00B          0.00B           ~     (all equal)
IPv4_inline-8                        0.00B          0.00B           ~     (all equal)
StdIPv6-8                            16.0B ± 0%     16.0B ± 0%      ~     (all equal)
IPv6-8                               16.0B ± 0%     16.0B ± 0%      ~     (all equal)
IPv4Contains-8                       0.00B          0.00B           ~     (all equal)
ParseIPv4-8                          0.00B          0.00B           ~     (all equal)
ParseIPv6-8                          48.0B ± 0%     48.0B ± 0%      ~     (all equal)
StdParseIPv4-8                       16.0B ± 0%     16.0B ± 0%      ~     (all equal)
StdParseIPv6-8                       16.0B ± 0%     16.0B ± 0%      ~     (all equal)
IPPrefixMasking/IPv4_/32-8           0.00B          0.00B           ~     (all equal)
IPPrefixMasking/IPv4_/17-8           0.00B          0.00B           ~     (all equal)
IPPrefixMasking/IPv4_/0-8            0.00B          0.00B           ~     (all equal)
IPPrefixMasking/IPv6_/128-8          0.00B          0.00B           ~     (all equal)
IPPrefixMasking/IPv6_/65-8           0.00B          0.00B           ~     (all equal)
IPPrefixMasking/IPv6_/0-8            0.00B          0.00B           ~     (all equal)
IPPrefixMasking/IPv6_zone_/128-8     16.0B ± 0%      0.0B       -100.00%  (p=0.008 n=5+5)
IPPrefixMasking/IPv6_zone_/65-8      16.0B ± 0%      0.0B       -100.00%  (p=0.008 n=5+5)
IPPrefixMasking/IPv6_zone_/0-8       16.0B ± 0%      0.0B       -100.00%  (p=0.008 n=5+5)
IPSetFuzz-8                         2.60kB ± 0%    2.60kB ± 0%      ~     (p=1.000 n=5+5)

name                              old allocs/op  new allocs/op  delta
StdIPv4-8                             1.00 ± 0%      1.00 ± 0%      ~     (all equal)
IPv4-8                                0.00           0.00           ~     (all equal)
IPv4_inline-8                         0.00           0.00           ~     (all equal)
StdIPv6-8                             1.00 ± 0%      1.00 ± 0%      ~     (all equal)
IPv6-8                                1.00 ± 0%      1.00 ± 0%      ~     (all equal)
IPv4Contains-8                        0.00           0.00           ~     (all equal)
ParseIPv4-8                           0.00           0.00           ~     (all equal)
ParseIPv6-8                           3.00 ± 0%      3.00 ± 0%      ~     (all equal)
StdParseIPv4-8                        1.00 ± 0%      1.00 ± 0%      ~     (all equal)
StdParseIPv6-8                        1.00 ± 0%      1.00 ± 0%      ~     (all equal)
IPPrefixMasking/IPv4_/32-8            0.00           0.00           ~     (all equal)
IPPrefixMasking/IPv4_/17-8            0.00           0.00           ~     (all equal)
IPPrefixMasking/IPv4_/0-8             0.00           0.00           ~     (all equal)
IPPrefixMasking/IPv6_/128-8           0.00           0.00           ~     (all equal)
IPPrefixMasking/IPv6_/65-8            0.00           0.00           ~     (all equal)
IPPrefixMasking/IPv6_/0-8             0.00           0.00           ~     (all equal)
IPPrefixMasking/IPv6_zone_/128-8      1.00 ± 0%      0.00       -100.00%  (p=0.008 n=5+5)
IPPrefixMasking/IPv6_zone_/65-8       1.00 ± 0%      0.00       -100.00%  (p=0.008 n=5+5)
IPPrefixMasking/IPv6_zone_/0-8        1.00 ± 0%      0.00       -100.00%  (p=0.008 n=5+5)
IPSetFuzz-8                           33.0 ± 0%      33.0 ± 0%      ~     (all equal)

Signed-off-by: David Anderson <dave@natulte.net>
Signed-off-by: Stefan Majer <stefan.majer@gmail.com>