Returning a 'var T' is not always safe

[Araq]

Returning a 'var T' does not mean it is save to return it:

  proc forward[T](x: var T): var T = result = x
  proc p(): var int = 
    var x: int
    # reject this call via alias analysis:
    result = forward(x)

[Araq]

This has now been fixed. RFC still open because it needs manual / documentation additions.