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/src/hash.cpp b/src/hash.cpp
new file mode 100644
index 0000000..ebba1d7
--- /dev/null
+++ b/src/hash.cpp
@@ -0,0 +1 @@
+#include <stdplus/hash.hpp>