C struct generator: have single header struct

[ThirteenFish]

Currently for each new data container we do something like this:

struct FooData {
    type field1;
    type field2;
}

struct FooMessage {
    char ID[4];
    uint8_t timestamp[6];
    uint16_t length;
    struct FooData data;
}

Instead we should have a single struct for the header and then either have struct FooMessage be a combination of the two structs, or have FooData be expanded inside FooMessage, or something.