feat: decoder CheckIntegrity now return two values

[muktihari]

• CheckIntegrity sequence completed and any error encountered. sequence completed is useful to know how many FIT sequences are valid so we can recover it.
• Reduce alloc on initDecodeHeaderOnce() by inlining the function and variable.
• Move header validity order, should be check FileHeader's Size and bytes 8-12 first so we can sure that it's a FIT file, then we validate the other values.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (854b1ce) 99.89% compared to head (a7faafc) 99.89%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #95      +/-   ##
==========================================
- Coverage   99.89%   99.89%   -0.01%     
==========================================
  Files          42       42              
  Lines        3841     3839       -2     
==========================================
- Hits         3837     3835       -2     
  Misses          4        4              

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