perf: optimize get_field

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Optimize existing function.

What changes are included in this PR?

Removed the per-row BooleanArray::slice allocation in map key lookup by scanning the key-comparison bitmap in place over each row's offset range.

Are these changes tested?

Existing tests.

Benchmark (criterion):

• get_field_map_1024_entries_16_last: 6.23% faster (base 42011ns -> cand 39394ns)
• get_field_map_1024_entries_4_missing: 12.267% faster (base 19070ns -> cand 16730ns)
• get_field_map_8192_entries_4_last: 8.122% faster (base 168450ns -> cand 154769ns)
• get_field_map_1024_entries_4_last: 11.166% faster (base 23229ns -> cand 20635ns)
• get_field_map_1024_entries_4_first: 15.396% faster (base 20877ns -> cand 17663ns)

Full criterion output:

get_field_map_1024_entries_4_first
                        time:   [17.664 µs 17.701 µs 17.756 µs]
                        change: [−15.807% −15.396% −14.964%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 6 outliers among 100 measurements (6.00%)
  4 (4.00%) high mild
  2 (2.00%) high severe

get_field_map_1024_entries_4_last
                        time:   [20.450 µs 20.499 µs 20.546 µs]
                        change: [−11.535% −11.166% −10.793%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 6 outliers among 100 measurements (6.00%)
  2 (2.00%) low mild
  4 (4.00%) high mild

get_field_map_1024_entries_16_last
                        time:   [39.406 µs 39.460 µs 39.516 µs]
                        change: [−6.9089% −6.2297% −5.5570%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 11 outliers among 100 measurements (11.00%)
  6 (6.00%) low severe
  2 (2.00%) high mild
  3 (3.00%) high severe

get_field_map_1024_entries_4_missing
                        time:   [16.755 µs 16.775 µs 16.796 µs]
                        change: [−12.675% −12.267% −11.888%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 6 outliers among 100 measurements (6.00%)
  4 (4.00%) high mild
  2 (2.00%) high severe

get_field_map_8192_entries_4_last
                        time:   [154.23 µs 154.92 µs 155.72 µs]
                        change: [−8.5962% −8.1217% −7.6240%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 12 outliers among 100 measurements (12.00%)
  11 (11.00%) high mild
  1 (1.00%) high severe

Are there any user-facing changes?

No