variant: Add equals operator helper

This adds a generic equality operator intended to make it
straightforward to compare a variant to other variants or base values.

Change-Id: If59296e650bfb43880931e146e4e50b3d8aaa38f
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/test/meson.build b/test/meson.build
index c9ee047..9c0580b 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -16,6 +16,7 @@
   'str/cexpr': [stdplus_dep, gtest_main_dep],
   'str/maps': [stdplus_dep, gmock_dep, gtest_main_dep],
   'util/cexec': [stdplus_dep, gtest_main_dep],
+  'variant': [stdplus_dep, gtest_main_dep],
   'zstring': [stdplus_dep, gtest_main_dep],
   'zstring_view': [stdplus_dep, gtest_main_dep],
 }