Add a function for creating a string from an address and length

Adds [string_from_ptr][string_from_ptr].  Fixes #97.

Note that ` char_array_as_string` can now be defined as follows:

```ocaml
let char_array_as_string a = CArray.(string_from_ptr (start a) (length a))
```

[string_from_ptr]: https://github.com/yallop/ocaml-ctypes/blob/7e2147db/src/ctypes/ctypes.mli#L465-L470