Prevent copying of Flowvar and Pending results

[mratsim]

Some assumptions of Weave is that there is a single consumer of a future result (Flowvar or Pending) and that it tried to consume that result only once.

While we can't prevent the latter at compile-time, we can prevent the first by only allowing the sink/move operation on the type and making copies a compile-time error:

type
  Flowvar[T] = object

proc `=`[T](dst: var Flowvar[T], src: Flowvar[T]) {.error: "copying not allowed".}

proc newFlowvar(T: typedesc): Flowvar[T] =
  discard

let x = newFlowvar(int)

However we get unresolved generic parameter at the moment due to upstream bug nim-lang/Nim#14315