improve performance of unique and unique_by

[itchyny]

Previously, unique and unique_by filters are implemented using
group_by and then map(.[0]). This commit re-implements in C, to
avoid unnecessary boxing of grouping, and improves the performance.

[wader]

Looks good! any ballpark idea how much faster and less memory usage?

[itchyny]

Good question, and I understand that it's important. However, I've measured it several times and it doesn't seem to make much difference. The difference between the measurements of the same command is larger than the difference before and after the improvement.

[itchyny]

Oh, I had some mistakes while profiling due to non-static executables. Performance is actually better. unique_by is now 60% faster and unique is 3 times faster.

 $ jq -nc '[range(100000)]' > /tmp/x.json                                                                 
 $ hyperfine --warmup 10 './jq-old "unique_by(.)[0]" /tmp/x.json' './jq-new "unique_by(.)[0]" /tmp/x.json'
Benchmark 1: ./jq-old "unique_by(.)[0]" /tmp/x.json
  Time (mean ± σ):     116.1 ms ±   1.1 ms    [User: 105.5 ms, System: 9.4 ms]
  Range (min … max):   114.8 ms … 119.6 ms    25 runs

Benchmark 2: ./jq-new "unique_by(.)[0]" /tmp/x.json
  Time (mean ± σ):      73.0 ms ±   0.5 ms    [User: 67.1 ms, System: 4.9 ms]
  Range (min … max):    72.4 ms …  74.3 ms    38 runs

Summary
  ./jq-new "unique_by(.)[0]" /tmp/x.json ran
    1.59 ± 0.02 times faster than ./jq-old "unique_by(.)[0]" /tmp/x.json

 $ hyperfine --warmup 10 './jq-old "unique[0]" /tmp/x.json' './jq-new "unique[0]" /tmp/x.json'
Benchmark 1: ./jq-old "unique[0]" /tmp/x.json
  Time (mean ± σ):     115.9 ms ±   1.9 ms    [User: 103.7 ms, System: 10.2 ms]
  Range (min … max):   113.1 ms … 120.7 ms    25 runs

Benchmark 2: ./jq-new "unique[0]" /tmp/x.json
  Time (mean ± σ):      37.2 ms ±   4.2 ms    [User: 31.5 ms, System: 4.0 ms]
  Range (min … max):    35.3 ms …  69.5 ms    75 runs

Summary
  ./jq-new "unique[0]" /tmp/x.json ran
    3.12 ± 0.36 times faster than ./jq-old "unique[0]" /tmp/x.json