Avoid parallel assignments

Parallel assignments in Ruby should only be used when in-place swapping is required, they are usually slower.

Before:
```
              parse:       31.5 (±25.4%) i/s -       1695 in  60.046965s
        parse & run:       14.6 (±20.5%) i/s -        785 in  59.995508s
```
```
              parse:       30.6 (±26.1%) i/s -       1674 in  60.048244s
        parse & run:       14.0 (±21.4%) i/s -        769 in  60.029016s
```

After:
```
              parse:       35.1 (±17.1%) i/s -       1992 in  60.001341s
        parse & run:       14.1 (±21.2%) i/s -        779 in  60.020685s
```
```
              parse:       35.5 (±16.9%) i/s -       2046 in  60.057296s
        parse & run:       15.4 (±13.0%) i/s -        877 in  60.023074s
```

@dylanahsmith @jasonhl