Add const / non-mutable variables

[Zettelkasten]

We still need to discuss some syntax for this. Maybe val/var as in Kotlin or so. Or simply const like here:

func foo() {
  const x: Int = 6
  x = 5  # not allowed, because = needs a reference to the target
}

These variables are then not alloca'ed as a Ref[T] but directly as T.
Then, all enforcements follow automatically as local variables are not special in any way currently.