[lowpri] serialization of containers-in-structs errors differently depending on the circumstances

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#### What version of the `csv` crate are you using?

1.1.5

#### Briefly describe the question, bug or feature request.
I was looking into how the csv crate implemented serde serialization/deserialization, and noticed a discrepancy.

In this case: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=0093262a4a6e3a775a70df76c8e9a91f
```
use std::error::Error;
use std::io;
use std::process;
use csv; // 1.1.5
use serde;

use serde::Serialize;

#[derive(Debug, Serialize)]
struct Record {
city: String,
region: String,
country: String,
population: Option,
v: Vec,
}

fn example() -> Result<(), Box> {
let mut wtr = csv::Writer::from_writer(io::stdout());

// When writing records with Serde using structs, the header row is written
// automatically.
wtr.serialize(Record {
city: "Southborough".to_string(),
region: "MA".to_string(),
country: "United States".to_string(),
population: Some(9686),
v: vec![1, 2, 3, 4],
})?;
wtr.serialize(Record {
city: "Northbridge".to_string(),
region: "MA".to_string(),
country: "United States".to_string(),
population: Some(14061),
v: vec![1, 2, 3, 4],
})?;
wtr.flush()?;
Ok(())
}

fn main() {
if let Err(err) = example() {
println!("error running example: {}", err);
process::exit(1);
}
}
```
outputs
```
city,region,country,population,verror running example: CSV write error: cannot serialize sequence container inside struct when writing headers from structs
```

It very clearly states that the problem is, serializing containers in a struct is impossible.

However
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=d9fe9283bd43cfce5fc9bebbde843ef3
```
wtr.serialize(Record {
city: "Southborough".to_string(),
region: "MA".to_string(),
country: "United States".to_string(),
population: Some(9686),
v: Some(vec![1, 2, 3, 4]),
})?;
wtr.serialize(Record {
city: "Northbridge".to_string(),
region: "MA".to_string(),
country: "United States".to_string(),
population: Some(14061),
v: Some(vec![1, 2, 3, 4]),
})?;
```
outputs
```
city,region,country,population,v
Southborough,MA,United States,9686,1,2,3,4error running example: CSV error: found record with 8 fields, but the previous record has 5 fields
```
showing that only the `Writer` notices the problem of "containers-in-structs" when it notices there are too many fields.

Similarly https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=275fdb4cf35a29bb47786590c699bcfc
shows that things like tuple structs fail late as well, and https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=275fdb4cf35a29bb47786590c699bcfc shows that top-level sequences only work if they are the exact same size.

Looking into the code of the serializer, and thinking about how csv works, it makes sense why this behavior is the way it is, however, I am wondering, do you think it would be worth it to restructure the serializer so it could give better error messages? I would be willing to look into this, unless you think its not possible/very difficult