`where` with final result at end

Possible syntaxes:
```python
out = where:
    x1 = a + b + c
    x2 = x + y + z
    x1 + x2

out = do:
    x1 = a + b + c
    x2 = x + y + z
    x1 + x2
```

Which should compile to the equivalent of:
```python
out = x1 + x2 where:
    x1 = a + b + c
    x2 = x + y + z
```