pkg/encoding: reduce allocations

[narqo]

This is a fairly quick and dirty change, that reduces some obvious allocations :)

name               old time/op    new time/op    delta
AnonymizeTrace-10    33.0µs ± 0%    31.4µs ± 0%   -4.66%  (p=0.008 n=5+5)

name               old alloc/op   new alloc/op   delta
AnonymizeTrace-10    9.82kB ± 0%   10.33kB ± 0%   +5.22%  (p=0.008 n=5+5)

name               old allocs/op  new allocs/op  delta
AnonymizeTrace-10      41.0 ± 0%      19.0 ± 0%  -53.66%  (p=0.008 n=5+5)

(updated the results after running the benchmarks on m1 mac)

[felixge]

Hey, thanks for this, I'll take a look when I get a chance. Right now the benchmark is biased towards processing lots of small traces, but for the implementation I probably want to bias towards efficiently processing large traces. I probably need to reflect the benchmark a bit to reflect that.

If I'm reading your code correctly, this would mostly help for processing lots of small traces, right?