dhcpv6: switch every Option to parse using FromBytes

[hugelgupf]

Adds

FromBytes([]byte) error

type to the Option interface.

This is a pre-requisite for doing either #487 or lazy option parsing. I'm experimenting with both, and unhappy with everything I come up with, so for now I want to merge this improvement.

[codecov]

Codecov Report

Base: 69.46% // Head: 69.55% // Increases project coverage by +0.09% 🎉

Coverage data is based on head (16b6fd4) compared to base (5989848).
Patch coverage: 85.35% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #491      +/-   ##
==========================================
+ Coverage   69.46%   69.55%   +0.09%     
==========================================
  Files          90       90              
  Lines        5737     5656      -81     
==========================================
- Hits         3985     3934      -51     
+ Misses       1571     1546      -25     
+ Partials      181      176       -5     

Impacted Files	Coverage Δ
dhcpv6/dhcpv6message.go	56.85% <0.00%> (+0.25%)	⬆️
dhcpv6/options.go	78.65% <57.89%> (+5.82%)	⬆️
dhcpv6/option_ntp_server.go	66.66% <68.00%> (+3.25%)	⬆️
dhcpv6/option_fqdn.go	83.33% <80.00%> (-0.88%)	⬇️
dhcpv6/option_4rd.go	83.90% <100.00%> (-0.71%)	⬇️
dhcpv6/option_archtype.go	100.00% <100.00%> (ø)
dhcpv6/option_bootfileparam.go	86.36% <100.00%> (+9.44%)	⬆️
dhcpv6/option_bootfileurl.go	100.00% <100.00%> (ø)
dhcpv6/option_clientid.go	100.00% <100.00%> (ø)
dhcpv6/option_clientlinklayeraddress.go	100.00% <100.00%> (ø)
... and 24 more

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