NonEmpty

• Added NonEmptyList
• Reorganised a little following the requirements of NEL and upstream
• Some List functions removed or altered in favour of typeclass versions:
  • filter (use Filterable, re-exported)
  • mapMaybe is now filterMap (from Filterable, re-exported)
  • mapWithIndex now being from FunctorWithIndex has its function arguments flipped (re-exported)
• Added functions
  • span, group, group', groupBy
  • unsnoc
  • filterM