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

[geekrelief]

That's awesome! Played around with it, and it can be as simple as this:

<https://play.nim-lang.org/#ix=3ZjH>
    
    
    import std / [macros, genasts]
    
    macro immut(n: typed) =
      result = genAst(name = ident(n.repr), n):
        let name = n
    
    proc doThing(i: var int) =
      immut i
      i = i + 1
      echo i+i
    
    var a = 100
    doThing(a)
    
    
    Run