Resource ownership rules when integrating with C++

These questions apply to both embedded JIT mode as well as to "as a shared library" mode.

On Codon side:
```
extern
def f1(s:str) -> str
```

On C++ side

```

struct CodonString {
seq_int_t len;
char *str;
};

extern "C" CodonString f2( CodonString s);
```

Questions:
1. C++ side invokes f1:
* Who owns string memory in incoming argument `s` ? If it's codon - can C++ "ref count" it and later unref or it must copy ?
* Who owns string memory of return string? C++ has generated this string by allocating it with `seq_alloc_atomic`. But who will release it and when?

2. Codon side invokes f2:
* Who owns string memory in `s`?
* Who owns string memory of return string?

These rules must be explained and documented.

Thanks!