str/buf: Add allocation reducing string buffer

Enables constructing strings with minimal allocations, and follows best
practices of using stack optimized for storing smaller string buffers.

Change-Id: If3b9e0d9e14147aadf282fe5ddeea8f7f9ef5837
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/test/meson.build b/test/meson.build
index 8403e0f..a92e411 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -13,6 +13,7 @@
   'pinned': [stdplus_dep, gtest_main_dep],
   'raw': [stdplus_dep, gmock_dep, gtest_main_dep],
   'signal': [stdplus_dep, gtest_main_dep],
+  'str/buf': [stdplus_dep, gtest_main_dep],
   'str/cat': [stdplus_dep, gtest_main_dep],
   'str/cexpr': [stdplus_dep, gtest_main_dep],
   'str/maps': [stdplus_dep, gmock_dep, gtest_main_dep],