Generate From<> rust conversions when the encoding is infallible

When converting from a child packet to a parent the encoding is often infallible, but the rust backend still generates TryFrom<> implementations instead of From<>.

For example:

```
packet Command {
op_code: 8,
_size_(_payload_),
_payload_,
}

packet OneCommand : Command (op_code = 1) {
x: 16,
y: 16,
}
```

Generates

```
impl TryFrom for Command {}
```

When the conversion can never fail.