variant: Add equals operator helper

This adds a generic equality operator intended to make it
straightforward to compare a variant to other variants or base values.

Change-Id: If59296e650bfb43880931e146e4e50b3d8aaa38f
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/variant.cpp b/src/variant.cpp
new file mode 100644
index 0000000..6af326c
--- /dev/null
+++ b/src/variant.cpp
@@ -0,0 +1 @@
+#include <stdplus/variant.hpp>