Support to convert int json to value

Hi

I was creating a native function which returns an int64 value and faced following error:
```
RUNTIME ERROR: Not a json type: 1234567890
```

After debugging this further, I found that we do not support converting int64 json to value (https://github.com/google/go-jsonnet/blob/master/interpreter.go#L934). Is there any reason we are not supporting int and int64 values? If not, I am happy to implement it.