Improves performance of reading large dbfs

[Maximkaaa]

This commit improves the way the values are read from the dbf files improving the speed of the file processing by almost 3 times.

The key points of improvement:

• Removes Vector::resize() call in read_string_of_len function. Using vec![0; len] instead gives significant improvement.

• Instead of using String::trim() on the read values (to remove spaces) we first skip the empty bytes, and then convert the value into the string. Removing trim calls improves performance almost doublefold.

• Using unchecked indexing on the vector when trimming spaces provides ~30% improvement in comparison to the safe version. Using unsafe here is not strictly necessary, but the logic in the unsafe block is quite straitforward so it should be ok.

Overall in my case reading a 500 MiB dbf file takes:

• 1 min 20 sec before change

• 34 sec after change

[tmontaigu]

Interesting

I'd like to find a 500Mb file to test out those improvements.

Removes Vector::resize() call in read_string_of_len function. Using vec![0; len] instead gives significant improvement.

Seems strange, I would have expected to two to be somewhat equivalent but that's cool

[Maximkaaa]

I did some more performance testing just to be sure. For it i used two files: 500 MB and 39 MB. The test code is just:

#[test]
fn large() {
    let mut reader = Reader::from_path("./tests/data/gis_osm_water_a_free_1.dbf").unwrap();
    let start = std::time::SystemTime::now();
    reader.read();
    let ellapsed = start.elapsed().unwrap().as_millis();
    assert_eq!(0, ellapsed);
}

The results are:

                                39 MB       500 MB

Without modification            1067 ms     69513 ms
.resize() -> vec![0; len]       851 ms      63046 ms
All improvements                521 ms      21756 ms

Both release and debug builds show similar performance improvement.

Unfortunately, I cannot provide the 500 MB file I use as it's commercial data. But the smaller one I downloaded from Geofabric OSM download. There are larger ones also, so you can probably find some files for testing (https://download.geofabrik.de/europe.html).

[Maximkaaa]

I also updated the PR to make fmt check pass. And only after that I noticed that it fails in the file I didn't originally change.