Type checker does not distinguish between let and letref

[edsko]

The following program makes it through the type checker without errors:

fun f(a : int) {
  let b = 1;
  b := 2;
  return a + b;
}

let comp main = read[int] >>> map f >>> write[int] 

In fact, it makes it all the way through to code generation, even after optimization turns this into

/*
let f(a: int32) =
      1 := 2;
      a+1
in
read >>>
map f >>>
write

[edsko]

Same for for bound variables (for i in ..).