String Interpolation

Try to answer to [https://github.com/golo-lang/golo-lang/issues/14]

So far it only interpolates identifiers. 
```
function variable_interpolation = {
  let theAnswer = 42
  require("The answer is ${theAnswer}" == "The answer is 42", "content must be equals")
}
```
Advices and thoughts are welcomed!

**TODO**
* Intepolate Expressions
* Documentation
* IR Dumping