Complex Types - structs

[mohitk05]

This PR implements struct data structures in Gom. structs can be defined as:

type Point = {
  x: int,
  y: int
}

and then initialised as:

let p = Point { x: 1, y: 2 };

Structs can be nested:

type Line = {
  p1: Point,
  p2: Point
}