improve the packet API

[marten-seemann]

We need a way to easily return an ICMP packet to the caller.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.96%. Comparing base (25cbbbc) to head (0ca27fd).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
conn.go	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   87.83%   87.96%   +0.13%     
==========================================
  Files           8        8              
  Lines         674      673       -1     
==========================================
  Hits          592      592              
+ Misses         63       62       -1     
  Partials       19       19              

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