Clean up object serialization API

[MaddieM4]

There's a bit of awkwardly duplicated code in document.serialize and event.serialize. Both of them should, ideally, accept a format object and pass it on to utils.serialize.

Will also need changes to the test suite.

[MaddieM4]

Another, very valid option is to remove the per-object convenience functions (which are somewhat redundant) and simply use the Utils.serialize function. I actually kinda prefer this, since it will also make it more consistent to serialize arbitrary objects in the serialization test suite in the future.