feat: optimise generic decodes

[nrwiersma]

This PR optimises the generic decode.

The benchmarked change is as follows:

name                      old time/op    new time/op    delta
SuperheroDecode-8            265ns ± 1%     254ns ± 2%   -4.10%  (p=0.000 n=10+10)
SuperheroEncode-8            201ns ± 1%     201ns ± 1%     ~     (p=0.946 n=9+9)
PartialSuperheroDecode-8     119ns ± 1%     118ns ± 0%   -0.86%  (p=0.003 n=9+9)
SuperheroGenericDecode-8    23.8µs ± 1%     2.1µs ± 0%  -91.23%  (p=0.000 n=10+10)
SuperheroGenericEncode-8     255ns ± 3%     260ns ± 2%   +1.91%  (p=0.004 n=10+10)
SuperheroWriteFlush-8        165ns ± 1%     164ns ± 1%   -0.60%  (p=0.008 n=9+10)

name                      old alloc/op   new alloc/op   delta
SuperheroDecode-8            47.0B ± 0%     47.0B ± 0%     ~     (all equal)
SuperheroEncode-8             112B ± 0%      112B ± 0%     ~     (all equal)
PartialSuperheroDecode-8     9.00B ± 0%     9.00B ± 0%     ~     (all equal)
SuperheroGenericDecode-8    8.37kB ± 0%    2.04kB ± 0%  -75.65%  (p=0.000 n=10+10)
SuperheroGenericEncode-8     80.0B ± 0%     80.0B ± 0%     ~     (all equal)
SuperheroWriteFlush-8        0.00B          0.00B          ~     (all equal)

name                      old allocs/op  new allocs/op  delta
SuperheroDecode-8             0.00           0.00          ~     (all equal)
SuperheroEncode-8             1.00 ± 0%      1.00 ± 0%     ~     (all equal)
PartialSuperheroDecode-8      0.00           0.00          ~     (all equal)
SuperheroGenericDecode-8       303 ± 0%        60 ± 0%  -80.20%  (p=0.000 n=10+10)
SuperheroGenericEncode-8      3.00 ± 0%      3.00 ± 0%     ~     (all equal)
SuperheroWriteFlush-8         0.00           0.00          ~     (all equal)

Fixes #347