Introduce encapsulating types & replace IPv4 payload_len with total_length

[codecov[bot]]

Codecov Report

Patch coverage: 99.35% and no project coverage change.

Comparison is base (c7a407b) 99.31% compared to head (499bc0c) 99.31%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #64 +/- ## ========================================= Coverage 99.31% 99.31% ========================================= Files 118 141 +23 Lines 31672 34881 +3209 ========================================= + Hits 31454 34643 +3189 - Misses 218 238 +20 ``` | [Impacted Files](https://app.codecov.io/gh/JulianSchmid/etherparse/pull/64?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Julian+Schmid) | Coverage Δ | | |---|---|---| | [etherparse/src/err/ip/header\_error.rs](https://app.codecov.io/gh/JulianSchmid/etherparse/pull/64?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Julian+Schmid#diff-ZXRoZXJwYXJzZS9zcmMvZXJyL2lwL2hlYWRlcl9lcnJvci5ycw==) | `100.00% <ø> (ø)` | | | [etherparse/src/err/ipv4/header\_error.rs](https://app.codecov.io/gh/JulianSchmid/etherparse/pull/64?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Julian+Schmid#diff-ZXRoZXJwYXJzZS9zcmMvZXJyL2lwdjQvaGVhZGVyX2Vycm9yLnJz) | `100.00% <ø> (ø)` | | | [etherparse/src/lib.rs](https://app.codecov.io/gh/JulianSchmid/etherparse/pull/64?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Julian+Schmid#diff-ZXRoZXJwYXJzZS9zcmMvbGliLnJz) | `100.00% <ø> (ø)` | | | [etherparse/src/link/ethernet2\_header.rs](https://app.codecov.io/gh/JulianSchmid/etherparse/pull/64?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Julian+Schmid#diff-ZXRoZXJwYXJzZS9zcmMvbGluay9ldGhlcm5ldDJfaGVhZGVyLnJz) | `100.00% <ø> (ø)` | | | [etherparse/src/packet\_filter.rs](https://app.codecov.io/gh/JulianSchmid/etherparse/pull/64?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Julian+Schmid#diff-ZXRoZXJwYXJzZS9zcmMvcGFja2V0X2ZpbHRlci5ycw==) | `100.00% <ø> (ø)` | | | [etherparse/src/packet\_builder.rs](https://app.codecov.io/gh/JulianSchmid/etherparse/pull/64?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Julian+Schmid#diff-ZXRoZXJwYXJzZS9zcmMvcGFja2V0X2J1aWxkZXIucnM=) | `97.50% <75.00%> (-2.50%)` | :arrow_down: | | [etherparse/src/internet/ipv4\_header\_slice.rs](https://app.codecov.io/gh/JulianSchmid/etherparse/pull/64?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Julian+Schmid#diff-ZXRoZXJwYXJzZS9zcmMvaW50ZXJuZXQvaXB2NF9oZWFkZXJfc2xpY2UucnM=) | `98.16% <89.85%> (-1.84%)` | :arrow_down: | | [etherparse/src/internet/ipv6\_exts.rs](https://app.codecov.io/gh/JulianSchmid/etherparse/pull/64?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Julian+Schmid#diff-ZXRoZXJwYXJzZS9zcmMvaW50ZXJuZXQvaXB2Nl9leHRzLnJz) | `98.49% <95.04%> (-1.24%)` | :arrow_down: | | [etherparse\_proptest\_generators/src/lib.rs](https://app.codecov.io/gh/JulianSchmid/etherparse/pull/64?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Julian+Schmid#diff-ZXRoZXJwYXJzZV9wcm9wdGVzdF9nZW5lcmF0b3JzL3NyYy9saWIucnM=) | `73.92% <95.12%> (-0.82%)` | :arrow_down: | | [etherparse/src/err/io/limited\_read\_error.rs](https://app.codecov.io/gh/JulianSchmid/etherparse/pull/64?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Julian+Schmid#diff-ZXRoZXJwYXJzZS9zcmMvZXJyL2lvL2xpbWl0ZWRfcmVhZF9lcnJvci5ycw==) | `100.00% <100.00%> (ø)` | | | ... and [65 more](https://app.codecov.io/gh/JulianSchmid/etherparse/pull/64?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Julian+Schmid) | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/JulianSchmid/etherparse/pull/64/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Julian+Schmid)

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