feat(core): add custom (de)serialization format & support

[imrn99]

Description

Scope: core

Type of change: feat

Content description:

• define a custom serialization format for combinatorial maps
  • the spec is detailed in a new UG page
• add corresponding implementations
  • a serialize methods to CMap2 and CMap3
  • a cmap_file method to CMapBuilder

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0.63291% with 314 lines in your changes missing coverage. Please review.

Project coverage is 82.47%. Comparing base (2701d38) to head (57ffe9f).

Files with missing lines	Patch %	Lines
honeycomb-core/src/cmap/builder/io.rs	0.00%	139 Missing ⚠️
honeycomb-core/src/cmap/dim3/serialize.rs	0.00%	85 Missing ⚠️
honeycomb-core/src/cmap/dim2/serialize.rs	0.00%	77 Missing ⚠️
honeycomb-core/src/cmap/builder/structure.rs	13.33%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #276      +/-   ##
==========================================
- Coverage   85.31%   82.47%   -2.84%     
==========================================
  Files          62       63       +1     
  Lines        9111     9427     +316     
==========================================
+ Hits         7773     7775       +2     
- Misses       1338     1652     +314     

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[imrn99]

TODO:

• finish processing the last todo!s
• add serialization test
• add deserialization test