Implement serialisation/de-serialisation to use with `mirai`

Maybe need custom serialisation/de-serialisation support in order to send to new processes via [`mirai`](https://mirai.r-lib.org/index.html).

```R
mirai::serial_config(
    c("ClassDefinition", "ClassInstance"),
    list(function(x) ???, function(x) ???),
    list(function(x) ???, function(x) ???)
)
```

Not sure if this is possible yet though. 