Round-trip failure with certain unsigned int64 values

Jsonnet produces surprising output for certain unsigned int64 values. For example, with this `test.jsonnet`:

```
{
a_uint: 18446744073709551615,
b_uint: 9223372036854775807
}
```

`jsonnet test.jsonnet` produces this output:

```
{
"a_uint": 18446744073709551616,
"b_uint": 9223372036854775808
}
```

Tagging Framework-R-D/phlex#243.

EDIT: Evidently this is because *all* numbers are 64-bit floats in Jsonnet.