Consider default values and accessors for optional fields

It may be useful if

```proto
message Child { ... }
message Msg {
Child field_name = 1;
}
```

generated

```rust
pub struct Msg {
field_name: Option
}

impl Msg {
// NEW!!
pub fn field_name(&self) -> &Child {
&self.field_name.unwrap_or(&Child::_DEFAULT_INSTANCE)
}

const _DEFAULT_INSTANCE : Msg = Default::default();
}

pub struct Child
```