perf: optimize Substring to work directly with strings instead of converting to runes

[d-enk]

• Rewrite Substring to iterate over string bytes directly, avoiding full []rune conversion
• Improve performance for long strings by only processing necessary portions
• Add comprehensive test cases for Unicode handling, invalid UTF-8, and edge cases
• Add BenchmarkSubstring to measure performance improvements
• Improve documentation with detailed parameter descriptions
• Handle invalid UTF-8 sequences by converting to []rune when needed

Bencstat:

                   │    old.txt    │               new.txt               │
                   │    sec/op     │    sec/op     vs base               │
Substring/{10_10}-4    558.85n ±  9%   39.75n ± 10%  -92.89% (p=0.000 n=8)
Substring/{50_50}-4    783.10n ±  6%   85.15n ±  5%  -89.13% (p=0.000 n=8)
Substring/{50_45}-4    773.30n ±  3%   126.5n ±  7%  -83.65% (p=0.000 n=8)
Substring/{-50_50}-4   794.00n ±  2%   177.6n ±  7%  -77.63% (p=0.000 n=8)
Substring/{-10_10}-4   542.85n ± 20%   41.82n ±  6%  -92.30% (p=0.000 n=8)
geomean               680.4n         79.52n        -88.31%

                   │  old.txt   │               new.txt                │
                   │    B/op    │   B/op    vs base                    │
Substring/{10_10}-4    432.0 ± 0%   0.0 ± 0%  -100.00% (p=0.000 n=8)
Substring/{50_50}-4    480.0 ± 0%   0.0 ± 0%  -100.00% (p=0.000 n=8)
Substring/{50_45}-4    464.0 ± 0%   0.0 ± 0%  -100.00% (p=0.000 n=8)
Substring/{-50_50}-4   480.0 ± 0%   0.0 ± 0%  -100.00% (p=0.000 n=8)
Substring/{-10_10}-4   432.0 ± 0%   0.0 ± 0%  -100.00% (p=0.000 n=8)

                   │  old.txt   │                new.txt                 │
                   │ allocs/op  │ allocs/op   vs base                    │
Substring/{10_10}-4    2.000 ± 0%   0.000 ± 0%  -100.00% (p=0.000 n=8)
Substring/{50_50}-4    2.000 ± 0%   0.000 ± 0%  -100.00% (p=0.000 n=8)
Substring/{50_45}-4    2.000 ± 0%   0.000 ± 0%  -100.00% (p=0.000 n=8)
Substring/{-50_50}-4   2.000 ± 0%   0.000 ± 0%  -100.00% (p=0.000 n=8)
Substring/{-10_10}-4   2.000 ± 0%   0.000 ± 0%  -100.00% (p=0.000 n=8)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.11%. Comparing base (ac8295b) to head (fbd209a).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #822      +/-   ##
==========================================
+ Coverage   92.03%   92.11%   +0.07%     
==========================================
  Files          32       32              
  Lines        4232     4259      +27     
==========================================
+ Hits         3895     3923      +28     
+ Misses        253      252       -1     
  Partials       84       84              

Flag	Coverage Δ
unittests	92.11% <100.00%> (+0.07%)	⬆️

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[d-enk]

Сompletely repeats the current behavior

But looks like here it's as if instead of

strings.ReplaceAll(str, "\x00", "")

was supposed to be

strings.ToValidUTF8(str, "")