Handle default case in the middle of the switch statement

[lucic71]

This PR adds support for handling code like:

int default_middle(int x) {
  int r = 0;
  switch (x) {
  case 1:
    r = 1;
    break;
  default: // default case in the middle of the switch statement
    r = 99;
    break;
  case 2:
    r = 2;
    break;
  }
  return r;
}

When translating this to rust, the default arm must be moved at the end of the match statement, otherwise all arms after the default one get ignored, making the following assertion fail assert(default_middle(2) == 2):

pub unsafe fn default_middle_4(mut x: i32) -> i32 {
    let mut r: i32 = 0;
    'switch: {
        let __match_cond = x;
        match __match_cond {
            v if v == 1 => {
                r = 1;
                break 'switch;
            }
            v if v == 2 => {
                r = 2;
                break 'switch;
            }
            _ => { // default branch moved at the end
                r = 99;
                break 'switch;
            }
        }
    };
    return r;
}

To implement this, VisitSwitchStmt iterates over all top level cases and defers the conversion of default arm after all non-default arms are converted.