Add support for size calculation when writing

[rain1]

Currently the serialization of json into object looks like this:

// template and obj definitions
const buffer = new ArrayBuffer(10);
const jbOut = new jBinary(buffer, template);
jbOut.write('parent', obj);

Problem is that if actual object is more than 10 bytes it gets: Uncaught RangeError: offset is outside the bounds of the DataView.

It would be cool if size calculation was included in jBinary so that I would not have to traverse the structure myself.