converted constant tables, support kres.type.TYPE1234

converted constant tables, support kres.type.TYPE1234

The difficulty with using structs as constant tables is that access
to non-existent fields throws an error. This is difficult to handle
without wrapping every access in a pcall, for example in predict module:

```
error: /usr/local/lib/kdns_modules/predict.lua:34: 'struct rr_type' has no member named 'TYPE65535'
```

So I converted the constant tables into regular Lua tables,
and added a metatable for RR types to allow looking up unnamed types,
in the TYPE%d format. Looking up non-existent fields will now
return nil instead of throwing an error.