Add pimps to List[(K, V)]: keys, values, mapValues, flatMapValues

[xavierguihot]

Hello,

This is a suggestion for a bunch of pimps on List[(K, V)]:

• keys which is .map(_._1)
• values which is .map(_._2)
• mapValues which is an alias for mapSecond
• flatMapValues which is the flatMap equivalent of mapValues

[stacycurl]

Nice addition, thanks.