Funny, we can cast ptr to var for proc arguments

[Stefan_Salewski]

This is fine of course:
    
    
    proc p1(pi: ptr int) =
      pi[] = 7
    
    proc p2(pi: ptr int) =
      p1(pi)
    
    var x = create(int)
    p2(x)
    echo x[]
    
    
    Run

But I did not expect that this would compile:
    
    
    proc p3(i: var int) =
      i = 13
    
    proc p4(pi: ptr int) =
      p3(cast[var int](pi))
    
    var y = create(int)
    p4(y)
    echo y[]
    
    
    Run

On the other hand, var proc parameters are basically pointers, so maybe I 
should be not surprised. That type of casting can be useful for C wrappers, 
when we have a C pointer and want to call a Nim proc with a var parameter...