Shadowing function arguments may cause null to surface

[marad]

Example problematic code:

fn foo(arg: int) {
  println(arg)
  val arg: string = "bar"
  println(arg)
}

This will print to console:

null
bar

Two issues here:

1. It shouldn't even compile since first println should see int type and require casting to string.
2. In first println the value should be 10.

[marad]

To fix that compiler would have to be structured differently. The easy way out is to simply not allow name shadowing.

One idea to explore is to detect shadowing and maybe use virtual compilation scope for the rest of the function body. No idea if this is easy or not.