Flat list to nested struct

[termoshtt]

From #131

Motivation

struct A {
  x: f64,
  y: f64,
}
struct B {
  z: f64,
  a: A,
}

In this case, we hope to deserialize B((1.0, 2.0, 3.0)) (in STEP) into B { z: 1.0, a: A { x: 2.0, y: 3.0 } } (In Rust)

[termoshtt]

This should be handled as internal mapping for complex entities, i.e. duplicated by #189