Spec and implement new tuple syntax and tuples with named fields

[mohammadfawaz]

As discussed during the design meeting:

• New tuple expression and type syntax: let t: { int, int, real } = {5, 6, 7.0 };
• Tuple index expressions remain the same
• Tuples can have named fields: let t: { x: int, y: int, z: real } = {5, 6, 7.0 };
• Tuples with named fields can be accessed in two ways: either via the field index: e.g. t.1, or via the field name: t.y.
• Tuple fields can be initialized with or without names. e.g. let t: { x: int, y: int, z: real } = { x: 5, y: 6, 7.0 };
• Tuple fields can only be initialized out of order if:
  • All tuple fields are named fields.
  • All tuple fields are initialized with their names.