Explicit nested struct serialization options

[Gusarich]

Currently, the compiler automatically determines whether to serialize a nested struct inlined in the same cell or as a reference, based on whether it fits. This proposal introduces a way for developers to explicitly specify the desired serialization mode.

For example:

struct B {
    c: Int;
    d: Int;
}

struct A {
    a: Int;
    b: Int;
    c: B as inline;  // Always inline
    d: B as ref;     // Always reference
}

With this feature:

• c will always be serialized in the same cell as A, while
• d will always be serialized as a reference, regardless of their sizes.

If the specified serialization mode is not feasible (e.g., a large struct is forced as inline but cannot fit in the cell), the compiler will throw a compilation error.