Import mapbox::variant in lieu of c++17 variant

See http://github.com/mapbox/variant.

Change-Id: Idbb2af80f0478188b652858d08817a5450630f0f
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/mapbox/README b/mapbox/README
new file mode 100644
index 0000000..d54e938
--- /dev/null
+++ b/mapbox/README
@@ -0,0 +1,3 @@
+This implementation of variant came from http://github.com/mapbox/variant.  As
+soon as GCC7 with C++17 support is available, we should move to the
+std::variant implementation instead.