added Look-and-say sequence

ported from the rust submission on rosettacode.org (ref https://github.com/Hoverbear/rust-rosetta/issues/170). Changed the function from working on [int] to working directly on String, because:

1. the task is essentially all string-based operations
2. using ints would overflow quite quickly (we should use biguint)
3. the task itself makes sense when thought more in general to work on strings. You could take "hello" and read it at "1h1e2l1o" (which this program actually outputs if "hello" is fed to the look_and_say function
sorry, I was doing this while you commented. It actually was needed because otherwise I was only breaking out of the while, not the for.
But the "while" wasn't needed:

    for ...
        while ... {
            continue // without a reference to the external for
                     // this was continuing the while and giving me a neverending loop...
        }

but actually I only needed to do this:
    for ... {
        if ... { continue }
    }