Reenable bin/vat

Add missing src/devices/index.js with named exports from the devices'
top-halves.

fixes Agoric#51

src/devices/index.js

@@ -0,0 +1,5 @@
+// Export all the default exports of the driver top-halves.
+export { default as buildChannel } from './channel';
+export { default as buildInbound } from './inbound';
+export { default as buildOutbox } from './outbox';
+export { default as buildSharedStringTable } from './sharedTable';