Introduce encapsulating types & replace IPv4 payload_len with total_length

[JulianSchmid]

No description provided.

[codecov]

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

@@            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	Coverage Δ
etherparse/src/err/ip/header_error.rs	100.00% <ø> (ø)
etherparse/src/err/ipv4/header_error.rs	100.00% <ø> (ø)
etherparse/src/lib.rs	100.00% <ø> (ø)
etherparse/src/link/ethernet2_header.rs	100.00% <ø> (ø)
etherparse/src/packet_filter.rs	100.00% <ø> (ø)
etherparse/src/packet_builder.rs	97.50% <75.00%> (-2.50%)	⬇️
etherparse/src/internet/ipv4_header_slice.rs	98.16% <89.85%> (-1.84%)	⬇️
etherparse/src/internet/ipv6_exts.rs	98.49% <95.04%> (-1.24%)	⬇️
etherparse_proptest_generators/src/lib.rs	73.92% <95.12%> (-0.82%)	⬇️
etherparse/src/err/io/limited_read_error.rs	100.00% <100.00%> (ø)
... and 65 more

... and 1 file with indirect coverage changes

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