Tentative struct literals grammar - try 1

[cjllanwarne]

Trying to try out / proof of concept the grammar change in #297 before it opens to a vote.

NB: I didn't try this on anything more complex than:

version development

struct Person {
  String name
  Int age
}

workflow foo {
  Person x = Person(name="harry", age=17)
  Array[Int] xs = range(x.age)
}

[cjllanwarne]

This PR is now out of date. Closing