Refactor Packet struct to match RFC format

[ffmiruz]

Refactor Packet struct to match RFC format

Remove PayloadOffset from Header.
Don't keep unnecessary full data bytes inside Packet.
Packet Marshal and Unmarshal method API stay the same.

Fixes #90

This PR do breaking changes as part of v2 plan.
Related to #88 #79

[codecov]

Codecov Report

Merging #119 (31e5b0c) into master (4e87540) will decrease coverage by 0.16%.
The diff coverage is 94.33%.

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
- Coverage   88.97%   88.80%   -0.17%     
==========================================
  Files          13       13              
  Lines         671      670       -1     
==========================================
- Hits          597      595       -2     
- Misses         56       57       +1     
  Partials       18       18              

Flag	Coverage Δ
go	88.80% <94.33%> (-0.17%)	⬇️
wasm	88.20% <92.45%> (-0.17%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packet.go	85.09% <94.33%> (-0.55%)	⬇️

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[Sean-Der]

This LGTM! Nice work @ffmiyo let me comb through and see if we have any other tech debt.

@cameronelliott @aler9 mind helping with reviews also? Sorry my availability is gonna be bad the next few weeks, so need to pull some others in :)

[cameronelliott]

This LGTM! Nice work @ffmiyo let me comb through and see if we have any other tech debt.

@cameronelliott @aler9 mind helping with reviews also? Sorry my availability is gonna be bad the next few weeks, so need to pull some others in :)

I'll keep an eye out for stuff that I'm able to help with.
@Sean-Der

[aler9]

Since

1. PayloadOffset and Raw are considered needless (Remove Header.PayloadOffset and Packet.Raw #90)
2. tests are unchanged

It's probably good to go.