don't use [deprecated] inferred return types for macros

Matches milessabin/shapeless@7b930bac943f4723b8b9e3cfaa7a8774063fc92d, for scala/scala@87979ad96f3a07354be4c15cdf35f71d1d4739cb, by way of [@milessabin's post](https://twitter.com/milessabin/status/410890401185148928).  Thanks Miles!