`IpPacket` rewrite

[thvdveld]

I rewrote IpPacket such that we can add hop-by-hop headers, fragment headers and routing headers to the representation. I discussed in #800 why I needed those changes.

The downside is that the IpPacket struct is now 264 bytes, and thus clippy complains about this.

[codecov[bot]]

Codecov Report

Merging #802 (2152c80) into main (aa5d887) will decrease coverage by 0.13%. The diff coverage is 46.87%.

:exclamation: Current head 2152c80 differs from pull request most recent head 618d7e3. Consider uploading reports for the commit 618d7e3 to get more accurate results

@@            Coverage Diff             @@
##             main     #802      +/-   ##
==========================================
- Coverage   79.65%   79.53%   -0.13%     
==========================================
  Files          71       72       +1     
  Lines       27786    27711      -75     
==========================================
- Hits        22134    22041      -93     
- Misses       5652     5670      +18     

Impacted Files	Coverage Δ
src/iface/interface/mod.rs	49.67% <2.63%> (+0.17%)	:arrow_up:
src/iface/ip_packet.rs	50.00% <50.00%> (ø)
src/iface/interface/sixlowpan.rs	74.93% <61.90%> (ø)
src/iface/interface/ipv4.rs	68.50% <93.75%> (+0.53%)	:arrow_up:
src/iface/interface/ipv6.rs	91.38% <100.00%> (+0.12%)	:arrow_up:

... and 4 files with indirect coverage changes

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[thvdveld]

I think that the size variant should not be an issue.

[thvdveld]

@Dirbaio I added the feature flags as discussed on Matrix.

[Dirbaio]

:rocket: thank you!