Support monadic operations (map, flatMap) on collection contexts

[holograph]

Basically, CollectionDslContext needs to be extended with map, flatMap, filter etc. to support syntax such as:

case class Element(value: Int)
case class Container(elements: Seq[Element])

val containerValidator = Validator[Container] { c =>
  // First element's value must be 0
  c.elements.headOption.each map { _.value } should be == 0
}

For future consideration:

• c.elements.headOption.each is validWith { _.value should be == 0 }
• c.elements startsWith { _.value should be == 0 }
• c.elements.head is validWith { ... } -- unlikely as would require heuristic wrapping of collection access

[buildreactive]

+1 would love to see this.