For loops

[mbenke]

This PR adds for loops of the form

        for(i=1;i<=10;i=i+1) { s = s + i;}

Declaring variables in the initialisation block is allowed

        for(let i=1;i<=10;i=i+1) { s = s + i;}

TBD: multiple statements in the init/post blocks

[Y-Nak]

This could cause a bug when shadowing happens inside for body.
e.g.,
code:

import std.{Num,Add,Sub,Eq,Ord,Bounded,Typedef,le};

contract C {
    function main() -> word {
        let x = 100;
        let i = 0;
        let s = 0;
        for(i=0;i<=0;i=i+1) { let x = 1; s = x; }
        return s;
    }
}

hull:

object C {
  code {

    function main () -> word {
      let x : word
      x := 100
      let i : word
      i := 0
      let s : word
      s := 0
      for ({ i := 0
           }; le$word(i, 0); { i := Add_add$word(i, 1)
                             }) { let x : word
                                  x := 1
                                  s := 100
                                }
      return s
}

[Y-Nak]

We probably need to disallow let in the for post clause in either the parser or the type checker.
Currently, the below code can be compiled without an error.

import std.{Num,Add,Sub,Eq,Ord,Bounded,Typedef,le};

contract C {
    function main() -> word {
        let i = 0;
        let s = 99;
        for(i=0;i<=0;let j=1) { s = j; i = i + 1; }
        return s;
    }
}

[Y-Nak]

This causes a duplicated let problem in yul.
e.g.,
code:

import std.{Num,Add,Sub,Eq,Ord,Bounded,Typedef,le};

contract Prefor {
    function main() -> word {
        let i = 100;
        let s = 0;
        for(let i=1;i<=10;i=i+1) { s = s + i; }
        return s;
    }
}

yul:

    function usr$main () -> _result {
      let i
      i := 100
      let s
      s := 0
      let i
      for {i := 1 let _v5 let _v6 _v5 := usr$le$word(i, 10)} _v5 {_v6 := usr$Add_add$word(i, 1) i := _v6 _v5 := usr$le$word(i, 10)}
      {let _v7
       _v7 := usr$Add_add$word(s, i)
       s := _v7}
      _result := s
      leave
    }

[mbenke]

@Y-Nak thanks for the thorough review. I tried to fix the issues you found, please look again.

[rodrigogribeiro]

Great work!

[Y-Nak]

LGTM!