API improvement: remove RowParser

`RowParser` is entirely unnecessary.

Rather than

```
SQL("SELECT * FROM pokemon").asList(RowParser { row =>
  Pokemon(row.string("name"), row.short("level"), row.longOption("trainer_id"))
})
```

we could instead have

```
SQL("SELECT * FROM pokemon").asList { row =>
  Pokemon(row.string("name"), row.short("level"), row.longOption("trainer_id"))
}
```

This would simplify both the API, and relate's code.