Improve access to port references in Rust target

[oowekyala]

Currently when a reaction references a port of a child reactor, the Rust target exposes a port with a special name, that is bound to the child's port. For reactor banks this is more problematic, since banks are flattened into a Vec of ports, and the structure of the bank is lost.

The C++ and C targets do not do this. They expose a view on the child instance that allows access to only the declared dependencies. Implementing this would make the Rust target more similar to the others and simplify interactions with banks´.

See also this comment:

Are other targets doing this differently?

I am not sure if I understand how this works in Rust, but in C++ we do not unroll the multiport. In the case of multiports within a bank, the reaction body can say precisely which port in which reactor instance it wants to access. If it wants to read all values, it needs two nested loops:

target Cpp
reactor Source {
  output[5] out: int;
  reaction (startup) -> out {= /* ... */ =}
}
main reactor {
  source = new[4] Source()
  reaction (source.out) {=
    for (int i{0}; i < source.size(); i++) {
      for (int j{0}; j < source[i].out.size(); j++) {
         int value = *source[i].out[j].get();
         // ....
      }
    }
  =}
}

Originally posted by @cmnrd in #1228 (comment)