Use int.to_bytes to speed up decoding

[keis]

Lifting part of #70

Did benchmarks with pytest -k 'test_decode_random' --benchmark-max-time=10 --benchmark-autosave reruns show quite some difference in runs on longer data but this change consistently beats the old method.

0006 is from current master, 0009 is with this patch

Name (time in us)                                  Min                   Max                  Mean              StdDev                Median                 IQR             Outliers           OPS            Rounds  Iterations
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
test_decode_random[8] (0009_09bbb05)            1.9319 (1.0)         43.0930 (1.0)          2.6994 (1.0)        1.6113 (1.0)          2.1090 (1.0)        0.2910 (3.73)   27233;32164  370,454.0210 (1.0)      239178           1
test_decode_random[8] (0006_578b01f)            3.2480 (1.68)        84.7890 (1.97)         4.0059 (1.48)       1.9595 (1.22)         3.4380 (1.63)       0.2950 (3.78)   17721;44521  249,633.8634 (0.67)     260437           1
test_decode_random[32] (0009_09bbb05)           4.7771 (2.47)       538.9240 (12.51)        5.5988 (2.07)       2.6364 (1.64)         4.9809 (2.36)       0.0780 (1.0)    55328;141858  178,609.6576 (0.48)     934847           1
test_decode_random[32] (0006_578b01f)           9.6110 (4.97)       534.5290 (12.40)       11.1709 (4.14)       4.0129 (2.49)         9.8980 (4.69)       0.3449 (4.42)   20132;69952   89,518.2111 (0.24)     339237           1
test_decode_random[256] (0009_09bbb05)         44.2620 (22.91)      563.8010 (13.08)       53.0820 (19.66)     17.8012 (11.05)       45.3601 (21.51)      7.5930 (97.35)    7471;9157   18,838.7910 (0.05)      85183           1
test_decode_random[256] (0006_578b01f)        153.7560 (79.59)      680.9130 (15.80)      190.0000 (70.39)     45.1139 (28.00)      176.1965 (83.55)     47.6551 (610.98)   4521;2708    5,263.1585 (0.01)      39546           1
test_decode_random[1024] (0009_09bbb05)       387.7309 (200.70)   1,133.3140 (26.30)      446.1720 (165.29)    77.0297 (47.81)      411.7080 (195.22)    49.8960 (639.71)   2529;2453    2,241.2879 (0.01)      20401           1
test_decode_random[1024] (0006_578b01f)     1,822.5090 (943.37)   4,125.0370 (95.72)    2,737.7037 (>1000.0)  403.5820 (250.47)   2,687.3620 (>1000.0)  614.8642 (>1000.0)     1391;6      365.2696 (0.00)       4109           1