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/meson.build b/src/meson.build
index 7c4dabf..c4662ad 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -47,6 +47,7 @@
   'flags.cpp',
   'handle/copyable.cpp',
   'handle/managed.cpp',
+  'hash.cpp',
   'pinned.cpp',
   'raw.cpp',
   'signal.cpp',