zstring: Add class

This is the most efficient wrapper for nul-terminated strings as it does
not care about determining string length. Conversions to / from
ztring_view are provided to make it trivial to upgrade.

Change-Id: Ic83626c53b95b851c3c4a13b1eded8f1e40539f0
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/test/meson.build b/test/meson.build
index ade258e..2a6645f 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -7,6 +7,7 @@
   'signal': [stdplus_dep, gtest_main_dep],
   'util/cexec': [stdplus_dep, gtest_main_dep],
   'util/string': [stdplus_dep, gtest_main_dep],
+  'zstring': [stdplus_dep, gtest_main_dep],
   'zstring_view': [stdplus_dep, gtest_main_dep],
 }