Improve handling of default fields

[dodomorandi]

We should improve the handling of default parameters during serialization/deserialization.
There are some similar situations that has a common ground:

• Fields that can be a T, a Vec<T> and that can be omitted
• Fields that get a specific value if omitted
• Fields for which the empty value corresponds to being omitted

We should really define at least a new type that supports Serde and that behaves in the same way for omitted and empty values, also supporting both singular and plural types.

This issue also relates to the possibility of avoiding allocations for plural types (i.e.: Vec, HashMap) with a single value.