Rewrite ICMP extensions

[gpotter2]

This PR:

• re-write icmp extensions to be buildable, and integrated in the ICMP packet. This is a a breaking change, at least for dissection
• fixes Wrong checksum calculation for Icmp Extension Header #1773
• fixes ICMP Extension Header wrong padding placement #4281

[codecov]

Codecov Report

Merging #4332 (4e5bdc8) into master (d8d24da) will increase coverage by 0.04%.
Report is 14 commits behind head on master.
The diff coverage is 85.04%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4332      +/-   ##
==========================================
+ Coverage   82.01%   82.05%   +0.04%     
==========================================
  Files         350      350              
  Lines       82225    82894     +669     
==========================================
+ Hits        67440    68022     +582     
- Misses      14785    14872      +87     

Files	Coverage Δ
scapy/contrib/mpls.py	93.18% <100.00%> (+0.32%)	⬆️
scapy/fields.py	92.39% <100.00%> (+<0.01%)	⬆️
scapy/layers/inet6.py	88.68% <100.00%> (+0.01%)	⬆️
scapy/utils.py	73.09% <100.00%> (-1.99%)	⬇️
scapy/contrib/icmp_extensions.py	0.00% <0.00%> (-50.59%)	⬇️
scapy/layers/inet.py	71.63% <87.50%> (+1.04%)	⬆️

... and 27 files with indirect coverage changes

[gpotter2]

@guedou @p-l- @polybassa This is ready for review & merge.

[guedou]

@gpotter2 do you want that the reporters of the issues confirm that this PR fix them? If not, this is ready to be merged.