perf: move to filter map from map and filter

[AdeAttwood]

Instead of doing two passes of all candidates using map then a filter, this uses filter_map so we are only doing one pass for the candidates. This alone is quite a significant improvement of around 7%

Output of ./scripts/bench 0.x

Benchmark 1: 0.x
  Time (mean ± σ):      2.373 s ±  0.138 s    [User: 10.617 s, System: 1.697 s]
  Range (min … max):    2.124 s …  2.577 s    10 runs

Benchmark 2: HEAD
  Time (mean ± σ):      2.206 s ±  0.133 s    [User: 10.061 s, System: 1.811 s]
  Range (min … max):    1.940 s …  2.433 s    10 runs

Summary
  HEAD ran
    1.08 ± 0.09 times faster than 0.x

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The percentage difference is -7.00%
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