Shouldn't serde adjacently tagged enums be supported in no_std environments?
Eg
```rust
#[derive(Serialize, Deserialize)]
#[serde(tag = "t", content = "c")]
enum Block {
    Para(Vec<Inline>),
    Str(String),
}
``` 

As far as i can find it should only be untagged representation that requires alloc? Or am i missing something?