Text support for grouped parameters

[zslayton]

Ion 1.1 allows templates to declare that a given template parameter accepts a group of expressions rather than a single one. For example:

// Definition
// The "hobbies" parameter accepts an expression group; in text, 
// the group is delimited by an enclosing `[]`.
(macro person (name [hobbies]) 
    {name: name, hobbies: (make_list hobbies))

// Invocation
(:person "Kunal" ["Bungee jumping", "Baking", "Bird watching"])

// Expansion
{
  name: "Kunal",
  hobbies: [
    "Bungee jumping",
    "Baking",
    "Bird watching"
  ]
}