Add a nicer way to construct strings.

So instead of

return "(" + v.x.toString() + ", " + v.y.toString() + ", " + v.z.toString() + ")"

we could write:

return s"(${v.x}, ${v.y}, ${v.z})"

I am not sure if we should add a special syntax for strings with interpolation (like the s prefix in the example above) or just allow interpolation in every string (and use \$ to write a real dollar sign).

We should also add a syntax for multiline strings, like:

let s = """This is a
           string with
           several lines"""

This thread is open for suggestions and discussion.