pref: improve marshal & encoding (sec: -39%, B: -70%, allocs: -91%)

[muktihari]

benchstat:

goos: darwin
goarch: amd64
pkg: github.com/muktihari/fit/encoder
cpu: Intel(R) Core(TM) i5-5257U CPU @ 2.70GHz
         │    old.txt    │               new.txt                │
         │    sec/op     │    sec/op     vs base                │
Encode-4   152.86m ± 33%   92.27m ± 26%  -39.64% (p=0.001 n=10)

         │    old.txt    │               new.txt                │
         │     B/op      │     B/op      vs base                │
Encode-4   10.301Mi ± 0%   3.074Mi ± 0%  -70.16% (p=0.000 n=10)

         │   old.txt    │               new.txt               │
         │  allocs/op   │  allocs/op   vs base                │
Encode-4   1200.1k ± 0%   100.0k ± 0%  -91.66% (p=0.000 n=10)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (77d9f23) 100.00% compared to head (48d72aa) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #63   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           25        25           
  Lines         2962      3007   +45     
=========================================
+ Hits          2962      3007   +45     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.