Add functionality to support setting a struct as another struct's field

[entropidelic]

Inside a hint, it may be necessary to assign a struct inside another struct. Consider the example:

struct Foo {
    a: Bar
}

struct Bar {
    b: felt
}

A hint may want to assign field a from Foo like so:

ids.foo.a = ids.bar

Where foo and bar are instances of the structs Foo and Bar respectively.

This is not supported yet, and if necessary, support for this should be added.