net/addr/subnet: Add class for representing a network subnet

This makes it possible to represent and IPv4/IPv6 CIDR network segment
via a class. Provides useful functions to compute base address and
determine if addresses are inside the subnet.

Change-Id: Ib9d01e28b6c8a28ccb622fef87b217fc96daf905
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/test/meson.build b/test/meson.build
index 9c0580b..01104c5 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -8,6 +8,7 @@
   'hash/tuple': [stdplus_dep, gtest_main_dep],
   'net/addr/ether': [stdplus_dep, gtest_main_dep],
   'net/addr/ip': [stdplus_dep, gtest_main_dep],
+  'net/addr/subnet': [stdplus_dep, gtest_main_dep],
   'numeric/endian': [stdplus_dep, gtest_main_dep],
   'pinned': [stdplus_dep, gtest_main_dep],
   'raw': [stdplus_dep, gmock_dep, gtest_main_dep],