util/str: Add string concatentation methods

Change-Id: I5caf8e0731eb9ac0f18b84d25256ea0068fab03c
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/meson.build b/src/meson.build
index a6e82b9..aadb68d 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -31,4 +31,5 @@
 
 install_headers(
   'stdplus/util/cexec.hpp',
+  'stdplus/util/string.hpp',
   subdir: 'stdplus/util')