With -safe-string, b.[i] <- 'c' is now Bytes.set b i 'c'

Actually this code : 
```ocaml
let () =
  let s = Bytes.of_string "foo" in
  s.[0] <- 'c';
  print_endline (Bytes.to_string s)
```

prints `File "test.ml", line 3, characters 2-14:
Warning 3: deprecated: String.set
Use Bytes.set instead.` even with `-safe-string` flag.
This pull request checks if `-safe-string` flag is set and in this case alias `<-` to `Bytes.set` instead of `String.set`.
This difference could (should?) be removed...