Lists of builtin categories

```
Sum. S ::= [Integer]  ;
separator Integer "+" ;
```
Test input:
```
1+2 +3 +  4+ 5
```
__haskell__  and __haskell-gadt__ do not print this correctly:
```
[Abstract Syntax]
Sum [1,2,3,4,5]

[Linearized tree]
1 2 3 4 5
```
The reason is that the printing classes for builtins do not define the `prtList` function.
