Add ||| operator for alternatives forming a shapeless Coproduct

[sirthias]

The | operator requires that the types of the two rules being "or-ed" are "compatible".
In most cases this is what you want, i.e. (a: Rule1[Int]) | (b: Rule1[String]) shouldn't compile.

Sometimes, however, it'd be convenient if there was a variant of | (e.g. called |||) which automatically created a Coproduct of it's operands, if required:

def a: Rule1[Int] = ...
def b: Rule1[String] = ...
def c = rule { a | b } // doesn't compile
def c: Rule1[Int :+: String :+: CNil] = rule { a ||| b }
def d: Rule1[Int] = rule { a ||| a }