Make lazy not a protected class

[micheltuu]

Otherwise people can't extend sjsonnet.Val.Builtin2 since they can't implement

  def evalRhs(
      arg1: sjsonnet.Lazy,
      arg2: sjsonnet.Lazy,
      ev: sjsonnet.EvalScope,
      pos: sjsonnet.Position): sjsonnet.Val = {

See example

[micheltuu]

@stephenamar-db can you help take a look? looks like I can't assign a reviewer