Mark abbrev, column, wrap and outdent in std.string as @safe and pure

`abbrev` is not a template function and `column`, `wrap` and `outdent` only take built-in string types.

`abbrev` cannot be marked as pure because it uses `std.utf.stride` which may throw exceptions.
Also, `column`, `wrap` and `outdent` cannot be marked as pure because they use `foreach` statement for string types which may throw exceptions.