Added leftBy and rightBy.
Added leftBy and rightBy.

Atry/shapeless,alexarchambault/shapeless,liff/shapeless,joescii/shapeless,lukasz-golebiewski/shapeless,lambdista/shapeless,clhodapp/shapeless,roberth/shapeless,wheaties/shapeless,triggerNZ/shapeless,fthomas/shapeless,malcolmgreaves/shapeless,TomasMikula/shapeless,khronnuz/shapeless,milessabin/shapeless,notxcain/shapeless,isaka/shapeless,tek/shapeless,retronym/shapeless