BigInt.toString

[Ruby The Roobster via Digitalmars-d-learn]

How exactly can one store the string representation of a BigInt?  
The seemingly obvious

```d
//...
dchar[] ret; //dchar[] is necessary for my project
//Assume that val is a BigInt with a value set earlier:
val.toString(ret, "%d");
//...
```
doesn't work.  I am using x86_64 windows with -m64, and 
v2.100.1(LDC2 1.30.0).