perf: optimize levenshtein

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Optimize existing expression.

What changes are included in this PR?

Added Myers' bit-parallel Levenshtein fast path for ASCII inputs whose shorter side fits in a 64-bit word, replacing the per-cell character-wise DP with a few bitwise ops per text character.

Are these changes tested?

Existing tests + new unit tests.

Benchmark (criterion):

• levenshtein_string [size=1024, str_len=32]: 83.81% faster (base 662708ns -> cand 107290ns)
• levenshtein_string [size=1024, str_len=8]: 54.904% faster (base 58165ns -> cand 26230ns)
• levenshtein_string [size=4096, str_len=32]: 83.79% faster (base 2640918ns -> cand 428103ns)
• levenshtein_string [size=4096, str_len=8]: 54.274% faster (base 227761ns -> cand 104146ns)

Full criterion output:

levenshtein size=1024/levenshtein_string [size=1024, str_len=8]
                        time:   [26.188 µs 26.230 µs 26.298 µs]
                        change: [−55.035% −54.904% −54.755%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 1 outliers among 10 measurements (10.00%)
  1 (10.00%) high severe
levenshtein size=1024/levenshtein_string [size=1024, str_len=32]
                        time:   [107.24 µs 107.29 µs 107.35 µs]
                        change: [−83.837% −83.810% −83.768%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 2 outliers among 10 measurements (20.00%)
  2 (20.00%) high mild

levenshtein size=4096/levenshtein_string [size=4096, str_len=8]
                        time:   [104.07 µs 104.15 µs 104.24 µs]
                        change: [−54.330% −54.274% −54.218%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 1 outliers among 10 measurements (10.00%)
  1 (10.00%) high mild
levenshtein size=4096/levenshtein_string [size=4096, str_len=32]
                        time:   [427.90 µs 428.10 µs 428.33 µs]
                        change: [−83.799% −83.790% −83.780%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 2 outliers among 10 measurements (20.00%)
  2 (20.00%) high mild

Are there any user-facing changes?

No

[github-actions]

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.93%. Comparing base (140c7c5) to head (7fefc46).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #23543   +/-   ##
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  Coverage   81.93%   81.93%           
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  Files        1136     1136           
  Lines      429152   429244   +92     
  Branches   429152   429244   +92     
=======================================
+ Hits       351633   351715   +82     
- Misses      56475    56482    +7     
- Partials    21044    21047    +3     

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