Implementation of planout interpreter in Golang

I don't understand this function, but it seems to be doing more stuff than it needs to be.  Also, given the way the compiler works now, the argument to this function should be called `value`, not `values`, and I should fix that in the Python and Hack implementations (goes to show how often we end up using that operator :)

```
values := evaluate(m["values"], s.params).([]mixed)