Add casts to make Readline.readline work again.

I couldn't get `icr` to compile on `master`:

```
in /Users/booch/Work/Projects/Crystal/crystal/src/readline.cr:8: argument 'prompt' of 'LibReadline#readline' must be Char*, not String
    line = LibReadline.readline(prompt)
                                ^~~~~~
```

This pull request fixes that, allowing `icr` to compile and run. (Although it won't run any code yet, due to not being able to find `"prelude"`.)