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/test/meson.build b/test/meson.build
index 901527d..98435bf 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -3,6 +3,7 @@
   'exception': [stdplus_dep, gtest_main_dep],
   'handle/copyable': [stdplus_dep, gtest_main_dep],
   'handle/managed': [stdplus_dep, gtest_main_dep],
+  'hash': [stdplus_dep, gtest_main_dep],
   'pinned': [stdplus_dep, gtest_main_dep],
   'raw': [stdplus_dep, gmock_dep, gtest_main_dep],
   'signal': [stdplus_dep, gtest_main_dep],