[SC-975] Add library to handle bytes storage slices

[k06a]

Apparently this library functionality will be implemented as a Solidity language feature in the future: ethereum/solidity#7423

Notes:

1. BytesStorage.Slice.offset and BytesStorage.Slice.length can be packed into a single 32-bytes word
2. BytesMemory.Slice.pointer and BytesMemory.Slice.length can be packed into a single 32-bytes word and stored in UDT on stack, not in memory

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2a5521f) 97.33% compared to head (0d7e407) 97.56%.
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
+ Coverage   97.33%   97.56%   +0.23%     
==========================================
  Files          12       14       +2     
  Lines         263      288      +25     
  Branches       60       62       +2     
==========================================
+ Hits          256      281      +25     
  Misses          7        7              

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