CSE machine: bug in for loops

[martin-henz]

let y = 0;
for (let i = (y = y + 1); i <= 1; i = i + 1) {
    display(i);
}

should result in one loop execution and return 1, but it results in no loop executions and returns undefined in the CSE machine. Explanation: the value expression is duplicated in interpreter.ts (530):

      control.push(
        ast.blockStatement(
          [
            init,
            ^ here
            ast.forStatement(
              ast.assignmentExpression(id, valueExpression, command.loc),
                                                 ^ and here
              test,
              update,

[martin-henz]

The fix is to stick to the specs:
https://docs.sourceacademy.org/source_3.pdf
which says for (name = name; ...)
to avoid the issue.