Customize hashing for hot maps

[udoprog]

With this we're seeing a 5-10% speedup compared to the default hash used for hash maps:

aoc_2020_1a             time:   [214.55 µs 215.35 µs 216.22 µs]
                        change: [-6.7605% -6.2809% -5.8364%] (p = 0.00 < 0.05)
                        Performance has improved.
aoc_2020_1b             time:   [757.62 µs 759.64 µs 762.21 µs]
                        change: [-12.110% -11.672% -11.268%] (p = 0.00 < 0.05)
                        Performance has improved.
aoc_2020_11a            time:   [315.06 ms 315.79 ms 316.59 ms]
                        change: [-5.5867% -5.2768% -4.9616%] (p = 0.00 < 0.05)
                        Performance has improved.
aoc_2020_19b            time:   [219.67 ms 220.34 ms 221.07 ms]
                        change: [-6.2175% -5.8469% -5.4795%] (p = 0.00 < 0.05)
                        Performance has improved.
brainfuck_hello_world   time:   [843.70 µs 845.93 µs 848.87 µs]
                        change: [-7.5888% -6.8126% -6.0949%] (p = 0.00 < 0.05)
                        Performance has improved.
brainfuck_hello_world2  time:   [8.9777 ms 8.9983 ms 9.0236 ms]
                        change: [-7.3782% -6.9379% -6.5155%] (p = 0.00 < 0.05)
                        Performance has improved.
brainfuck_fib           time:   [47.993 ms 48.117 ms 48.275 ms]
                        change: [-10.628% -9.6902% -8.7642%] (p = 0.00 < 0.05)
                        Performance has improved.
brainfuck_loopity       time:   [7.7490 ms 7.7620 ms 7.7760 ms]
                        change: [-6.7243% -6.3566% -6.0391%] (p = 0.00 < 0.05)
                        Performance has improved.
fib_15                  time:   [346.42 µs 346.97 µs 347.93 µs]
                        change: [-5.5870% -4.9540% -4.3864%] (p = 0.00 < 0.05)
                        Performance has improved.
fib_20                  time:   [3.8800 ms 3.8835 ms 3.8873 ms]
                        change: [-5.3582% -5.2084% -5.0557%] (p = 0.00 < 0.05)
                        Performance has improved.
external_functions      time:   [570.02 ns 570.28 ns 570.60 ns]
                        change: [-3.6023% -3.1358% -2.6129%] (p = 0.00 < 0.05)
                        Performance has improved.