Unicode support and char improvments

* we can convert from integers to chars and back
* source files and input are assumed to be utf8 encoded

Two tiny examples:

```
user => (str \u269b)
"⚛"
user => (seq (map (str char) (range 1024)))
(" " "!" "\"" "#" "$" "%" "&" ... "ϸ" "Ϲ" "Ϻ" "ϻ" "ϼ" "Ͻ" "Ͼ" "Ͽ")
```