Consider replacing/augmenting MutableStructFields with an idiomatic builder class

[popematt]

The StructElement.update() method accepts a Consumer<MutableStructFields>, which has Collection/Map-like APIs. This leads to less-than-ideal code, such as this:

StructElement updated = original.update(fields -> { 
    fields.set("last_updated_date", ionTimestamp(Instant.now());
    fields.set("scores", orginal.get("scores").asStruct().update(scoreFields -> {
        scoreFields.add("ping_pong", ionInt(200)); 
        scoreFields.set("billiards", ionInt(30)); 
        scoreFields.remove("lawn_bowling");
    }));
});

If we had a StructElementBuilder with proper builder-style APIs (i.e. that return this) and specialized functions for modifying fields that have a container value, then we could write code like this instead:

StructElement updated = original.update(fields -> fields
    .set("last_updated_date", ionTimestamp(Instant.now())
    .updateStruct("scores", scores -> scores
        .add("ping_pong", ionInt(200))
        .set("billiards", ionInt(30))
        .remove("lawn_bowling")));

Formatting quibbles aside, the second example is more concise because we can chain together a series of add, set, and remove calls.