perf: Avoid allocation in the hot path for struct JSON serialization

[ChayimFriedman2]

This improves the following benchmark in 50%:

use criterion::{black_box, criterion_group, criterion_main, Criterion};
use polars::prelude::*;

fn to_json(df: &mut DataFrame) -> Vec<u8> {
    let mut json = Vec::new();
    JsonWriter::new(&mut json)
        .with_json_format(JsonFormat::Json)
        .finish(df)
        .unwrap();
    json
}

fn my_benchmark(c: &mut Criterion) {
    const SIZE: i32 = 10_000;
    let mut df = df![
        "a" => (0..SIZE).collect::<Vec<_>>(),
        "b" => (0..SIZE).map(|v| v.to_string()).collect::<Vec<_>>(),
    ]
    .unwrap();

    c.bench_function("to_json", |b| b.iter(|| to_json(black_box(&mut df))));
}

criterion_group!(benches, my_benchmark);
criterion_main!(benches);

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.13%. Comparing base (802d6c8) to head (4a9748f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15449      +/-   ##
==========================================
- Coverage   81.14%   81.13%   -0.02%     
==========================================
  Files        1362     1362              
  Lines      174846   174848       +2     
  Branches     2531     2531              
==========================================
- Hits       141881   141860      -21     
- Misses      32481    32504      +23     
  Partials      484      484              

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[ritchie46]

Thanks a lot. Painful to have that. :/