Support optional struct fields (as opposed to nullable)

[latos]

Optional means the field can be absent (not null).

[timbod7]

Would a hypothetical:

struct X
{
    optional T value;
};

be equivalent to

struct X
{
    Nullable<T> value = null;
}

along with a change to the serialiser that left out fields which have default values?

[timbod7]

See https://tim.dockerz.net/posts/2020-04-17-optionality-in-adl.html for details