perf: optimize parse_string_to_decimal (30-40% faster)

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Optimize existing expression.

What changes are included in this PR?

Replaced six redundant scans plus two str::parse calls with a single validating byte pass and direct i128 digit accumulation, gated the seven inf/nan string comparisons behind a first-byte check, and swapped 10_i128.pow for a const power-of-ten table.

How are these changes tested?

Existing tests.

Benchmark (criterion):

• decimal_38_10: 41.516% faster (base 265533ns -> cand 155293ns)
• decimal_18_2: 36.846% faster (base 302661ns -> cand 191142ns)
• decimal_38_10: 40.979% faster (base 266661ns -> cand 157386ns)
• decimal_18_2: 36.564% faster (base 305466ns -> cand 193776ns)
• decimal_38_10: 40.66% faster (base 264940ns -> cand 157214ns)
• decimal_18_2: 36.607% faster (base 303296ns -> cand 192269ns)

Full criterion output:

cast_string_to_decimal/legacy/decimal_38_10
                        time:   [156.99 µs 157.35 µs 157.96 µs]
                        change: [−41.202% −40.979% −40.792%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 10 outliers among 100 measurements (10.00%)
  2 (2.00%) low severe
  1 (1.00%) low mild
  1 (1.00%) high mild
  6 (6.00%) high severe
cast_string_to_decimal/legacy/decimal_18_2
                        time:   [193.54 µs 193.64 µs 193.77 µs]
                        change: [−36.658% −36.564% −36.469%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 10 outliers among 100 measurements (10.00%)
  3 (3.00%) low severe
  1 (1.00%) low mild
  2 (2.00%) high mild
  4 (4.00%) high severe

cast_string_to_decimal/ansi/decimal_38_10
                        time:   [155.19 µs 155.26 µs 155.35 µs]
                        change: [−41.605% −41.516% −41.425%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 14 outliers among 100 measurements (14.00%)
  4 (4.00%) low severe
  3 (3.00%) low mild
  5 (5.00%) high mild
  2 (2.00%) high severe
cast_string_to_decimal/ansi/decimal_18_2
                        time:   [191.08 µs 191.15 µs 191.25 µs]
                        change: [−36.908% −36.846% −36.785%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 8 outliers among 100 measurements (8.00%)
  1 (1.00%) low mild
  3 (3.00%) high mild
  4 (4.00%) high severe

cast_string_to_decimal/try/decimal_38_10
                        time:   [156.94 µs 157.14 µs 157.42 µs]
                        change: [−40.772% −40.660% −40.556%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 14 outliers among 100 measurements (14.00%)
  6 (6.00%) low mild
  3 (3.00%) high mild
  5 (5.00%) high severe
cast_string_to_decimal/try/decimal_18_2
                        time:   [192.19 µs 192.30 µs 192.46 µs]
                        change: [−36.697% −36.607% −36.524%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 3 outliers among 100 measurements (3.00%)
  1 (1.00%) high mild
  2 (2.00%) high severe