Add 'ether_payload' & 'ip_payload' methods to SlicedPacket types

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (4a71be5) 99.73% compared to head (4445dcf) 99.58%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #88 +/- ## ========================================== - Coverage 99.73% 99.58% -0.16% ========================================== Files 151 151 Lines 41839 27601 -14238 ========================================== - Hits 41728 27486 -14242 - Misses 111 115 +4 ```

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