Skip bitmap presence bits when pack and unpack message

[alovak]

closes #254

This PR adds the IsBitmapPresenceBit method to the Bitmap, allowing us to ignore fields that have the same index as bits, which indicate the presence of the next bitmap. By using this method, we can skip these fields when packing and unpacking a message.

[alovak]

this PR does not fix the same issue for composites. I'll fix it as well.

[codecov-commenter]

Codecov Report

Patch coverage: 71.42% and project coverage change: +0.05% 🎉

Comparison is base (026d31c) 73.43% compared to head (5d4c821) 73.48%.

❗ Current head 5d4c821 differs from pull request most recent head c324965. Consider uploading reports for the commit c324965 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #255      +/-   ##
==========================================
+ Coverage   73.43%   73.48%   +0.05%     
==========================================
  Files          43       43              
  Lines        2270     2282      +12     
==========================================
+ Hits         1667     1677      +10     
- Misses        373      374       +1     
- Partials      230      231       +1     

Files Changed	Coverage Δ
field/composite.go	83.22% <60.00%> (ø)
field/bitmap.go	71.05% <75.00%> (+0.29%)	⬆️
message.go	71.30% <100.00%> (+0.52%)	⬆️

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