Reworked layer slicing to include payloads & add lax parsing methods

[JulianSchmid]

Implements https://github.com/JulianSchmid/etherparse/issues/46

Lax Parsing with invalid length fields:

• IpHeader::from_slice
  • [x] update doc
• IpHeader::from_slice_lax
  • [x] implementation
  • [x] tests
• IpHeader::from_v4_slice_lax
  • [x] implementation
  • [x] tests
• IpHeader::from_v6_slice_lax
  • [x] implementation
  • [x] tests
• IpSlice
  • [x] implement
  • [x] document
  • [x] replace packet slicing return types
  • [x] Replace InternetSlice with an alias.
• IpSlice::from_ip_slice
  • [x] implement
  • [x] test
• IpSlice::from_ip_slice_lax
  • [x] implement
  • [x] test
• From<Ipv6Slice> for IpSlice
  • [x] implement
  • [x] test
• From<Ipv4Slice> for IpSlice
  • [x] implement
  • [x] test
• Ipv4Header:: from_slice
  • [x] add hint in doc to use Ipv4Slice if correct payload seperation is needed
• Ipv4HeaderSlice::from_slice
  • [x] add hint in doc to use Ipv4Slice if correct payload seperation is needed
• Ipv4Slice::from_slice_lax
  • [x] implementation
  • [x] tests
• Ipv6Header:: from_slice
  • [x] add hint in doc to use Ipv6Slice if correct payload separation is needed
• Ipv6Slice::from_slice
  • [x] check
  • [x] update doc with hint if needed
• Ipv6Slice::from_slice_lax
  • [x] implementation
  • [x] tests

Lax Parsing with support for LenErrors & ContentErrors in later headers

• Ipv6ExtensionsSlice::from_slice_lax
  • [x] implementation
  • [x] doc
  • [x] test
• Ipv4ExtensionsSlice::from_slice_lax
  • [x] implementation
  • [x] doc
  • [x] test
• Ipv6Extensions::from_slice_lax
  • [x] implementation
  • [x] doc
  • [x] test
• Ipv4Extensions::from_slice_lax
  • [x] implementation
  • [x] doc
  • [x] test
• LaxIpSlice::from_ip_slice
  • [x] Allow for errors in extension header
  • [x] implementation
  • [x] doc
  • [x] test
• LaxIpv4Slice::from_slice
  • [x] Allow for errors in extension header
  • [x] implementation
  • [x] doc
  • [x] test
• LaxIpv6Slice::from_slice
  • [x] Allow for errors in extension header
  • [x] implementation
  • [x] doc
  • [x] test
• IpHeader::from_slice
  • [x] update doc
• IpHeader::from_slice_lax
  • [x] Allow for errors in extension header
  • [x] implementation
  • [x] doc
  • [x] test
• IpHeader::from_v4_slice_lax
  • [x] Allow for errors in extension header
  • [x] implementation
  • [x] doc
  • [x] test
• IpHeader::from_v6_slice_lax
  • [x] Allow for errors in extension header
  • [x] implementation
  • [x] doc
  • [x] test
• [x] UdpSlice::from_slice_lax

Provide Lax Packet parsing

• LaxSlicedPacket::from_ethernet
  • [x] implementation
  • [x] Allow for errors in headers of lower levels
  • [x] tests
• LaxSlicedPacket::from_ether_type
  • [x] implementation
  • [x] Allow for errors in headers of lower levels
  • [x] tests
• LaxSlicedPacket::from_ip
  • [x] implementation
  • [x] Allow for errors in headers of lower levels
  • [x] tests
• LaxPacketHeaders::from_ethernet_slice
  • [x] Allow for errors in headers of lower levels
  • [x] implementation
  • [x] doc
  • [x] tests
• LaxPacketHeaders::from_ether_type
  • [x] Allow for errors in headers of lower levels
  • [x] implementation
  • [x] doc
  • [x] tests
• LaxPacketHeaders::from_ip_slice
  • [x] Allow for errors in headers of lower levels
  • [x] implementation
  • [x] doc
  • [x] tests
• ICMP Tests
  • [x] reactivate tests that need lax parsing
• [x] Move LenSource out of error

[codecov[bot]]

Codecov Report

Attention: 154 lines in your changes are missing coverage. Please review.

Comparison is base (4c6c586) 0.00% compared to head (3a1da93) 97.74%.

:exclamation: Current head 3a1da93 differs from pull request most recent head e440268. Consider uploading reports for the commit e440268 to get more accurate results

Files	Patch %	Lines
etherparse/src/sliced_packet.rs	66.40%	126 Missing :warning:
etherparse/src/internet/ipv6_exts.rs	98.92%	16 Missing :warning:
etherparse/src/packet_filter.rs	98.49%	10 Missing :warning:
etherparse/src/err/ip/headers_error.rs	98.55%	1 Missing :warning:
etherparse/src/packet_builder.rs	99.93%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #71 +/- ## =========================================== + Coverage 0 97.74% +97.74% =========================================== Files 0 135 +135 Lines 0 38410 +38410 =========================================== + Hits 0 37542 +37542 - Misses 0 868 +868 ```

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