Remove all `Option<HashMap<K,V>>` in favor of `HashMap<K,V>` with an empty default

[LegNeato]

It is more ergonomic to use an empty HashMap and set a default when serializing and deserializing instead of an Option so that code doesn't always have to check before using a field.

Because creating a default allocates, ideally we would put this behind a feature flag and let users decide if they prefer the ergonomics or (slight) perf increase.

[srijs]

IIRC creating an empty HashMap does not perform any heap allocations. Or are you referring to something else when you say ‚default‘?

[LegNeato]

Nope, I assumed it allocated but never checked. In that case I think this would strictly be a win.