Small improvement discussion: Use `let` to lock variable previously declared as `var`

[geekrelief]

Nevermind, you can shadow a var in a proc, but you can't do it at the top 
level.j
    
    
    proc test(i: var int) =
      let i = i # works in a  proc
      #i = i + 1
      echo i
    
    var a = 100
    test(a)
    let a = a # doesn't work here
    
    
    Run