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/include/meson.build b/include/meson.build
index d7cf82b..5909aa0 100644
--- a/include/meson.build
+++ b/include/meson.build
@@ -16,6 +16,7 @@
   'stdplus/pinned.hpp',
   'stdplus/raw.hpp',
   'stdplus/signal.hpp',
+  'stdplus/str/buf.hpp',
   'stdplus/str/cat.hpp',
   'stdplus/str/cexpr.hpp',
   'stdplus/str/maps.hpp',