Make while, for and if return values

`if` is essentially a `while` loop that is executed once at most, however return values for `if` and `while` are inconsistent.

If returns the value of its last statement as its value:
```
🦄 .editor
// Entering editor mode (^D to finish, ^C to cancel)
if(true) {
let b = 4
}

int ask(if(call(get('true')),block(let('b',4)),block()))
4
```

However while always returns null:
```
🦄 .editor
// Entering editor mode (^D to finish, ^C to cancel)
let b = 2
while(b<4) {
b = b+1
}

empty ask(let('b',2),while(call(get('<'),get('b'),4),block(assign('b',call(get('+'),get('b'),1)))))
null
🦄
```

The issue provides editor examples for if and while and names while, for, and if as the affected constructs. Locate the implementations and existing tests for these control-flow forms, then compare their expression results against the examples. Done means the three constructs have consistent, documented return-value behavior and the relevant tests pass.

Written by the indexing model from the issue text.