Implementation of planout interpreter in Golang

If I am understanding your code correctly this doesn't do what it's supposed to do.  See the original implementation of set, and also keep in mind:

```
colors = ['#aa2200', '#22aa00', '#0022aa'];
x = uniformChoice(choices=colors, unit=userid); # 'x' used as salt
y = uniformChoice(choices=colors, unit=userid); # 'y' used as salt, generally != x
z = uniformChoice(choices=colors, unit=userid, salt='x'); # same value as x
```