perf: Change PlSmallStr impl from Arc<str> to compact_str

[nameexhaustion]

• compact_str for small-string in-lining, and after testing also preferred against Arc<str> / kstring<arc> as the atomic ref-counting performed badly under contention
• Replace PlSmallStr::const_default() with more ergonomic PlSmallStr::EMPTY

[nameexhaustion]

We are now const fn for from_static

[ritchie46]

I marked this as a perf change. 😎

[codecov]

Codecov Report

Attention: Patch coverage is 83.84798% with 68 lines in your changes missing coverage. Please review.

Project coverage is 79.87%. Comparing base (c03e760) to head (54cd6ea).
Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-python/src/expr/rolling.rs	15.38%	22 Missing ⚠️
...polars-core/src/chunked_array/from_iterator_par.rs	50.00%	5 Missing ⚠️
...es/polars-core/src/chunked_array/comparison/mod.rs	94.20%	4 Missing ⚠️
...tes/polars-core/src/chunked_array/from_iterator.rs	76.47%	4 Missing ⚠️
...rates/polars-plan/src/dsl/function_expr/boolean.rs	0.00%	4 Missing ⚠️
...ates/polars-core/src/chunked_array/iterator/mod.rs	70.00%	3 Missing ⚠️
crates/polars-core/src/frame/row/av_buffer.rs	91.42%	3 Missing ⚠️
crates/polars-core/src/chunked_array/ops/apply.rs	0.00%	2 Missing ⚠️
...s-core/src/frame/group_by/aggregations/dispatch.rs	71.42%	2 Missing ⚠️
crates/polars-core/src/series/any_value.rs	90.47%	2 Missing ⚠️
... and 17 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18508      +/-   ##
==========================================
+ Coverage   79.86%   79.87%   +0.01%     
==========================================
  Files        1501     1501              
  Lines      202029   201777     -252     
  Branches     2868     2868              
==========================================
- Hits       161342   161174     -168     
+ Misses      40140    40056      -84     
  Partials      547      547              

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[ritchie46]

Internal benchmarks:

use std::hint::black_box;
use std::sync::Arc;
use std::time::Duration;

use compact_str::CompactString;

fn bench_multithread_clone<T: Clone + Send + 'static>(n_iters: usize, n_threads: usize, value: T) -> Duration {
    let start = std::time::Instant::now();
    let mut threads = Vec::with_capacity(n_threads);
    for _ in 0..n_threads {
        let value = value.clone();
        threads.push(
            std::thread::spawn(move || {
                for _ in 0..n_iters {
                    black_box(value.clone());
                }
            })
        );
    }
    for t in threads {
        t.join().unwrap();
    }
    start.elapsed()
}

fn main() {
    let n_iters = 10_000_000;
    let n_threads = 10;
    for logsz in 0..11 {
        let sz = 1 << logsz;
        let s = "a".repeat(sz);
        println!("String   {sz}: {:?}", bench_multithread_clone(n_iters, n_threads, s.clone()));
        println!("Arc<str> {sz}: {:?}", bench_multithread_clone(n_iters, n_threads, Arc::from_iter(s.chars())));
        println!("Compact  {sz}: {:?}", bench_multithread_clone(n_iters, n_threads, CompactString::from(&s)));
    }
}

High contention

String   1: 310.708458ms
Arc<str> 1: 10.11404825s
Compact  1: 15.558041ms
String   2: 327.036084ms
Arc<str> 2: 10.694496084s
Compact  2: 16.020833ms
String   4: 323.366625ms
Arc<str> 4: 10.955285917s
Compact  4: 17.379417ms
String   8: 328.070042ms
Arc<str> 8: 10.536749875s
Compact  8: 15.1295ms
String   16: 348.985125ms
Arc<str> 16: 11.969632458s
Compact  16: 16.202709ms
String   32: 254.559125ms
Arc<str> 32: 9.809960167s
Compact  32: 290.558875ms
String   64: 274.858916ms
Arc<str> 64: 9.887851958s
Compact  64: 272.698ms
String   128: 245.200375ms
Arc<str> 128: 10.31820525s
Compact  128: 249.990667ms
String   256: 261.957209ms
Arc<str> 256: 10.237664458s
Compact  256: 279.541459ms
String   512: 728.768542ms
Arc<str> 512: 10.320912292s
Compact  512: 763.411584ms
String   1024: 423.475666ms
Arc<str> 1024: 10.892088625s
Compact  1024: 512.338875ms

Lower contention

for _ in 0..n_iters {
    black_box(value.clone());
    for i in 0..1000 {
        black_box(i*i);
    }
}

String   1: 540.059708ms
Arc<str> 1: 1.389195917s
Compact  1: 415.176167ms
String   2: 525.745792ms
Arc<str> 2: 1.368962625s
Compact  2: 432.936209ms
String   4: 503.196ms
Arc<str> 4: 1.385971s
Compact  4: 493.812583ms
String   8: 497.884583ms
Arc<str> 8: 1.467783334s
Compact  8: 455.944667ms
String   16: 492.412459ms
Arc<str> 16: 1.386792791s
Compact  16: 454.659ms
String   32: 481.999208ms
Arc<str> 32: 1.336161917s
Compact  32: 523.770375ms
String   64: 472.040334ms
Arc<str> 64: 1.38562975s
Compact  64: 460.261959ms
String   128: 489.277875ms
Arc<str> 128: 1.448168959s
Compact  128: 481.207792ms
String   256: 486.657875ms
Arc<str> 256: 1.338470875s
Compact  256: 475.626292ms
String   512: 553.985458ms
Arc<str> 512: 1.365395s
Compact  512: 510.286834ms
String   1024: 481.878875ms
Arc<str> 1024: 1.444221125s
Compact  1024: 521.473291ms