`StringprepTablesReader` into a model that can be used at runtime.

This format is chosen to make it easy to validate that these tables are consistent with the RFC.

```
cd okhttp/src/test/resources/okhttp3/internal/idn/
ls rfc3454.*.txt | xargs -n 1 -I {} bash -c "echo {} ; cat {}" > okhttp_tables.txt
curl https://www.rfc-editor.org/rfc/rfc3454.txt > rfc3454.txt
diff rfc3454.txt okhttp_tables.txt | less

 [kotlin]: https://kotlinlang.org/
 [okhttp3_pro]: https://raw.githubusercontent.com/square/okhttp/master/okhttp/src/main/resources/META-INF/proguard/okhttp3.pro
 [okhttp_312x]: https://github.com/square/okhttp/tree/okhttp_3.12.x
 [okhttp]: https://square.github.io/okhttp/
 [okio]: https://github.com/square/okio