Add `@JsonWrapped`

It would be nice to have the inverse of `@JsonUnwrapped`.

This would enable serializing this class:

```
class Gene {
String symbol;
String chrId;
String chrName;
}
```

To this:

```
{
"symbol" : "X",
"chr" : {
"id" : "Y",
"name": "chr1"
}
}
```