Improve IPAddress::toFullyQualified() CPU performance

Improve IPAddress::toFullyQualified() CPU performance

Summary:
Currently IPAddress::toFullyQualified() is fairly slow for IPv6.

Change here implements more lightweight in_addr/in6_addr to string functions.

I also added a benchmark for comparison with inet_ntop.

This makes IPAddressV6::toFullyQualified() significantly faster than inet_ntop, and makes IPAddressV4::str() ~20ns faster than previous impementation (previously ~80ns).

Previous benchmark:
============================================================================
folly/test/IPAddressBenchmark.cpp               relative  time/iter  iters/s
============================================================================
ipv4_to_string_inet_ntop                                   238.91ns    4.19M
ipv4_to_fully_qualified                          289.96%    82.39ns   12.14M
----------------------------------------------------------------------------
ipv6_to_string_inet_ntop                                   780.72ns    1.28M
ipv6_to_fully_qualified                           51.11%     1.53us  654.59K
============================================================================

With this change:
============================================================================
folly/test/IPAddressBenchmark.cpp               relative  time/iter  iters/s
============================================================================
ipv4_to_string_inet_ntop                                   238.06ns    4.20M
ipv4_to_fully_qualified                          364.76%    65.26ns   15.32M
----------------------------------------------------------------------------
ipv6_to_string_inet_ntop                                   770.74ns    1.30M
ipv6_to_fully_qualified                          791.63%    97.36ns   10.27M
============================================================================

Test Plan:
fbconfig folly/test:network_address_test folly/test:network_address_benchmark
fbmake runtests_opt

Reviewed By: simpkins@fb.com

Subscribers: ps, bmatheny

FB internal diff: D1477925

Tasks: 4832974