Support Nested more convenient

Right now it can be done using separate arrays:
```rust
#[derive(Debug, Row, Serialize, Deserialize)]
struct MyRowOwned {
no: u32,
#[serde(rename = "nested.a")]
a: Vec,
#[serde(rename = "nested.b")]
b: Vec,
}
```

However it can be more convenient to detect following pattern:
```rust
#[derive(Debug, Row, Serialize, Deserialize)]
struct MyRowOwned {
no: u32,
nested: Vec,
}

#[derive(Debug, Row, Serialize, Deserialize)]
struct Nested {
a: f64,
b: f64,
}
```

The issue names no file, test, or entry point; start by locating the Row derive and the existing separate-array handling. Compare the requested nested Vec representation with the current mapping behavior, and consider the work complete when the nested form is supported without breaking the existing form.

Written by the indexing model from the issue text.