suboptimal inference for `nextind`, `prevind` on `AbstractString`

The `nextind` and `prevind` doc strings specify the return type as `Int` for any `AbstractString`, but inference doesn't know that even in a fresh REPL session:

```julia-repl
julia> Base.infer_return_type(prevind, Tuple{AbstractString, Vararg})
Any

julia> Base.infer_return_type(nextind, Tuple{AbstractString, Vararg})
Any
```

Probably we could add a type assertion or two.