SUSHI should (maybe) warn or error when people use `.value` on primitives

For example, if a user does:

```
Instance: MyPatient
InstanceOf: Patient
* address[0].line[0].value = "123 Easy Street"
```

Sushi does not complain, but it creates a weird instance:
```
{
"resourceType": "Patient",
"id": "MyPatient",
"address": [
{
"_line": [
{
"value": "123 Easy Street"
}
]
}
]
}
```

I suspect this is because `line` is a `string`, and the _formal_ definition of `string` _does_ have a `value` property (it's just supposed to be implicit in the JSON serialization).

If a user does this, SUSHI should either:
* emit a warning but render the JSON _correctly_
* or emit an error