Array splatting

[LightAndLight]

let
  a = 1
  b = 2
  rest = [3, 4, 5]
in
  [a, b, ..rest]

Together with #82, this gets arrays to a similar level of expressiveness as records. Arrays are 'homogeneous products', so it's consistent to provide similar packing/unpacking operations.

Tasks

• [ ] Implement feature
• [ ] Add golden tests
• [ ] Document grammar and behaviour in reference