<= doesn't work

This is embarrassing. Dlang should have a highly performant, yet extremely easy to use (no type casting) JSON library.

It shouldn't be that hard, just opXXX + templates.
```dlang
unittest {
    JSONValue data = toJSONValue(`{"key": 5}`);
    assert(data["key"] >= 4 && data["key"] <= 6)
}
```