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
The `'ext` lifetime of the loop could be removed to simplify things.