Update netaddr which made all structs opaque, which is good

[majst01]

also move json (de)serialization to separate file

[codecov-commenter]

Codecov Report

Merging #61 (aa99184) into master (45aa14d) will increase coverage by 1.90%.
The diff coverage is 84.50%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
+ Coverage   77.11%   79.02%   +1.90%     
==========================================
  Files           5        6       +1     
  Lines         472      553      +81     
==========================================
+ Hits          364      437      +73     
- Misses         57       62       +5     
- Partials       51       54       +3     

Impacted Files	Coverage Δ
sql.go	61.25% <62.50%> (-4.73%)	⬇️
prefix.go	80.16% <86.66%> (+1.54%)	⬆️
json.go	87.87% <87.87%> (ø)
ipam.go	100.00% <0.00%> (ø)
postgres.go	71.42% <0.00%> (+1.42%)	⬆️
memory.go	92.45% <0.00%> (+1.54%)	⬆️

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