function Context(stack, global, options, blocks, templateName) {
    if(stack !== undefined && !(stack instanceof Stack)) {
      stack = new Stack(stack);
    }
    this.stack = stack;
    this.global = global;
    this.options = options;
    this.blocks = blocks;
    this.templateName = templateName;
  }
Decide very naively if something is a Stream.
@param elem {*} object to inspect
@return {Boolean} is `elem` a Stream?