[feature] Fix #1095 #1104 Implement full IPv4 header and fragmentation

[aaronddavies]

Fixes #1095 and #1104.

Changes to operational code:

• Add the missing IPv4 fixed length fields. They are now a part of the Ipv4Repr struct so that they are no longer dropped when message traffic goes through smoltcp.
• Add support for options. This implemented as a fixed size byte array to the IPv4Repr struct. The header length field determines how many bytes of the options array are to be included when emitting from the struct. Bench tests showed that a the raw array had better performance than using Option<[u8]>.
• Filter options across fragments for outgoing packets.
  1. Options are persisted across fragments if the copy bit in the type octet is set.
  2. Option lengths are preserved according to the length octet following the type octet, with the exception of padding octets.
• Capture the options from an incoming packet. The options of the first incoming fragment of a packet stream are captured as the representative set of options for the final assembled packet, which is to be delivered to the sockets. NOTE: An options capture error in this process will cause the entire packet to be dropped.
• Fragment packet data in multiples of 8 octets according to RFC 791. The fragment offset for packet fragments is measured in units of 8 octets, therefore payloads must be fragmented in multiples of 8. This adds logic in two places: 1) where the first fragment is sent immediately, and 2) where each successive fragment is sent from the queue.

Changes to unit tests:

• Update all test IPv4 repr definitions to include the init of the rest of the header fields. This boilerplate is a significant portion of the line changes in this PR.
• Fix unit tests that don't require fragmentation to not specify fragmentation in their test packet headers
• Use a config-if block for choosing the right IpvX struct repr
• Fix some places to only import related modules if features require them
• Add unit tests for the new changes listed above

NOTE: This PR builds on #1094 and so those changes are also incorporated.

Reference:

1. RFC 791
2. IPv4 Parameters - Options

Bench tests before & after

current HEAD main (39cd44e)
test wire::bench_emit_ipv4 ... bench:           8.27 ns/iter (+/- 0.52)
test wire::bench_emit_ipv6 ... bench:           1.11 ns/iter (+/- 0.01)
test wire::bench_emit_tcp  ... bench:          29.89 ns/iter (+/- 0.20)
test wire::bench_emit_udp  ... bench:          25.04 ns/iter (+/- 0.18)


this branch (1bce638)
test wire::bench_emit_ipv4 ... bench:           9.44 ns/iter (+/- 0.54)
test wire::bench_emit_ipv6 ... bench:           1.11 ns/iter (+/- 0.01)
test wire::bench_emit_tcp  ... bench:          29.90 ns/iter (+/- 0.40)
test wire::bench_emit_udp  ... bench:          25.06 ns/iter (+/- 0.44)

[codecov]

Codecov Report

❌ Patch coverage is 94.38202% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.96%. Comparing base (39cd44e) to head (30b6272).

Files with missing lines	Patch %	Lines
src/wire/icmpv4.rs	0.00%	16 Missing ⚠️
src/wire/ipv4.rs	92.94%	11 Missing ⚠️
src/iface/interface/ipv4.rs	93.75%	2 Missing ⚠️
src/iface/fragmentation.rs	99.34%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1108      +/-   ##
==========================================
+ Coverage   80.45%   80.96%   +0.50%     
==========================================
  Files          81       81              
  Lines       24461    24951     +490     
==========================================
+ Hits        19681    20202     +521     
+ Misses       4780     4749      -31     

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