[Spec] sink parameter mutation

[mratsim]

It seems like sink parameter can be mutated.

This is actually very useful to me as for message passing a proc might want to take ownership of a message, update it and sink it into a channel. A var param would be needed otherwise and wouldn't be able to express the sink semantics.

i.e. this compiles

proc foo(x: sink int) =

  x += 1
  echo x

proc main() =
  var z = 3
  foo(z)


main()

However, this mutability of sink parameter is not clearly specified in the destructors spec.
If it's intentional, it should be added, if not it should be removed (and a sink var would be useful).

[mratsim]

Linked/duplicate of #12340