Optimize alignment and padding of structs

[mlegner]

There are specific representation and alignment rules for structs that affect the memory efficiency of our code. This is particularly pronounced if we use the C representation at some point (e.g., to enable C bindings and wrappers). Specifically, the ordering of fields becomes relevant for the C representation.

In general, having a hierarchy of structs (structs in structs) may be beneficial from a conceptual point of view but substantially less memory-efficient than a flattened struct.