`String.index_from` (among others) is unsafe

[v-gb]

This segfaults:

let _ = ignore (Base.String.index_from "" (-1000000000) 'c' : _ option)

Most other String.*index* functions are broken in this way, and also by accepting indices greater than the length of the string, whereas String.{r,}index_from_exn detect both kinds of bad inputs.

[nmatschke]

Oops. We'll fix this and respond once done.