Add several macro attributes#26

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oknozor wants to merge 3 commits intodylanmckay:masterfrom

Commits on Jul 19, 2021

Add fixed length attribute
oknozor
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Commits on Jul 20, 2021

Make length prefix usable more that once
oknozor
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Commits on Jul 21, 2021

Add skip_if macro attribute read implementation for struct named field
oknozor
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