Use vec instead of HashMap for tmp frames

[hackaugusto]

Split from #355 for a smaller changeset / easier review.

This changes merge and flow to use a Vec instead of a HashMap:

collapse/dtrace/1       time:   [2.8601 ms 2.8605 ms 2.8610 ms]
                        change: [−2.6285% −2.5659% −2.5035%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 4 outliers among 100 measurements (4.00%)
  1 (1.00%) low mild
  2 (2.00%) high mild
  1 (1.00%) high severe
Benchmarking collapse/dtrace/32: Warming up for 3.0000 s
Warning: Unable to complete 100 samples in 5.0s. You may wish to increase target time to 7.4s, enable flat sampling, or reduce sample count to 50.
collapse/dtrace/32      time:   [1.4648 ms 1.4695 ms 1.4744 ms]
                        thrpt:  [894.22 MiB/s 897.20 MiB/s 900.06 MiB/s]
                 change:
                        time:   [−3.7081% −3.1143% −2.4975%] (p = 0.00 < 0.05)
                        thrpt:  [+2.5615% +3.2144% +3.8509%]
                        Performance has improved.
Found 7 outliers among 100 measurements (7.00%)
  1 (1.00%) low severe
  1 (1.00%) low mild
  4 (4.00%) high mild
  1 (1.00%) high severe

collapse/perf/1         time:   [6.0919 ms 6.0961 ms 6.1005 ms]
                        change: [−1.2866% −1.1898% −1.0983%] (p = 0.00 < 0.05)
                        Performance has improved.
Benchmarking collapse/perf/32: Warming up for 3.0000 s
Warning: Unable to complete 100 samples in 5.0s. You may wish to increase target time to 9.9s, enable flat sampling, or reduce sample count to 40.
collapse/perf/32        time:   [1.9667 ms 1.9717 ms 1.9767 ms]
                        thrpt:  [1.4790 GiB/s 1.4828 GiB/s 1.4866 GiB/s]
                 change:
                        time:   [−2.7162% −2.2773% −1.8490%] (p = 0.00 < 0.05)
                        thrpt:  [+1.8839% +2.3304% +2.7921%]
                        Performance has improved.
Found 2 outliers among 100 measurements (2.00%)
  1 (1.00%) low mild
  1 (1.00%) high mild

sample/collapse         time:   [2.9114 ms 2.9139 ms 2.9165 ms]
                        change: [+0.8133% +0.9177% +1.0046%] (p = 0.00 < 0.05)
                        Change within noise threshold.

     Running benches/flamegraph.rs (target/release/deps/flamegraph-af6050c0ec451745)
Gnuplot not found, using plotters backend
flamegraph/flamegraph   time:   [16.217 ms 16.288 ms 16.360 ms]
                        change: [−7.6264% −7.0837% −6.5656%] (p = 0.00 < 0.05)
                        Performance has improved.