Substitute HashMap for BtreeMap in family

[spoutn1k]

Updates #128, fixes #125.

     Running benches/baseline.rs (/dev/shm/cargo/release/deps/baseline-81cd5bfd359a4ac9)
Gnuplot not found, using plotters backend
counter                 time:   [1.3984 ns 1.4016 ns 1.4051 ns]
                        change: [-1.2161% -0.8192% -0.4271%] (p = 0.00 < 0.05)
                        Change within noise threshold.
Found 8 outliers among 100 measurements (8.00%)
  2 (2.00%) low mild
  6 (6.00%) high mild

counter via family lookup
                        time:   [4.6534 ns 4.6551 ns 4.6572 ns]
                        change: [-57.571% -57.505% -57.441%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 10 outliers among 100 measurements (10.00%)
  5 (5.00%) high mild
  5 (5.00%) high severe

     Running benches/family.rs (/dev/shm/cargo/release/deps/family-adb364bb53eacb5d)
Gnuplot not found, using plotters backend
Benchmarking counter family with [(&'static str, &'static str)] label set: Collecting 100 samples in estimated 5.0000 s (480M iteratiocounter family with [(&'static str, &'static str)] label set
                        time:   [10.177 ns 10.268 ns 10.353 ns]
                        change: [-69.157% -69.052% -68.940%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 13 outliers among 100 measurements (13.00%)
  7 (7.00%) low severe
  4 (4.00%) high mild
  2 (2.00%) high severe

Benchmarking counter family with Vec<(&'static str, &'static str)> label set: Collecting 100 samples in estimated 5.0000 s (344M iteracounter family with Vec<(&'static str, &'static str)> label set
                        time:   [14.458 ns 14.469 ns 14.478 ns]
                        change: [-64.730% -64.632% -64.536%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 13 outliers among 100 measurements (13.00%)
  5 (5.00%) low severe
  8 (8.00%) high severe

counter family with Vec<(String, String)> label set
                        time:   [35.154 ns 35.173 ns 35.192 ns]
                        change: [-35.624% -35.495% -35.369%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 6 outliers among 100 measurements (6.00%)
  3 (3.00%) high mild
  3 (3.00%) high severe

counter family with custom type label set
                        time:   [4.6477 ns 4.6488 ns 4.6499 ns]
                        change: [-70.297% -70.224% -70.157%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 9 outliers among 100 measurements (9.00%)
  4 (4.00%) high mild
  5 (5.00%) high severe

     Running benches/encoding/text.rs (/dev/shm/cargo/release/deps/text-299cebf5df4a656a)
Gnuplot not found, using plotters backend
encode                  time:   [16.280 ms 16.310 ms 16.341 ms]
                        change: [-8.1091% -7.7785% -7.4542%] (p = 0.00 < 0.05)
                        Performance has improved.