Add IEEE 802.1AE (also known as MACsec) support

[JulianSchmid]

Implements #80

[codecov]

Codecov Report

Attention: Patch coverage is 98.71550% with 47 lines in your changes missing coverage. Please review.

Project coverage is 99.37%. Comparing base (397e512) to head (1ccaf44).
Report is 29 commits behind head on master.

Files with missing lines	Patch %	Lines
etherparse/src/lax_sliced_packet.rs	96.31%	19 Missing ⚠️
etherparse/src/compositions_tests.rs	87.83%	18 Missing ⚠️
etherparse/src/packet_headers.rs	99.33%	3 Missing ⚠️
etherparse/src/lax_sliced_packet_cursor.rs	98.11%	2 Missing ⚠️
etherparse/src/err/len_error.rs	0.00%	1 Missing ⚠️
etherparse/src/lax_packet_headers.rs	99.78%	1 Missing ⚠️
etherparse/src/link/linux_sll_payload_slice.rs	50.00%	1 Missing ⚠️
etherparse/src/sliced_packet.rs	99.83%	1 Missing ⚠️
etherparse/src/test_packet.rs	98.21%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
+ Coverage   99.25%   99.37%   +0.12%     
==========================================
  Files         177      185       +8     
  Lines       31788    33971    +2183     
==========================================
+ Hits        31551    33759    +2208     
+ Misses        237      212      -25     

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