hash: Add function for efficiently combining hashes

You can now call stdplus::hashMulti(...) with multiple arguments and get
a single resulting hash value.

Change-Id: I82d91f3372daeea941f6b9e8d57a224b5806527d
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/include/meson.build b/include/meson.build
index c3bfdfe..358bc32 100644
--- a/include/meson.build
+++ b/include/meson.build
@@ -6,6 +6,7 @@
   'stdplus/flags.hpp',
   'stdplus/handle/copyable.hpp',
   'stdplus/handle/managed.hpp',
+  'stdplus/hash.hpp',
   'stdplus/pinned.hpp',
   'stdplus/raw.hpp',
   'stdplus/signal.hpp',