Remove unsafe code

[basbossink]

This PR contains a trivial change to remove the unsafe block from src/lib.rs.

[JohnPeel]

You can't index str with an integer, because they are UTF-8.

(ArrayString used here, also does not give an interface to access individual bytes, as it makes the same assumptions on codepage)

error[E0277]: the type `str` cannot be indexed by `{integer}`