refactor!(core): delete the io feature from the core crate

[imrn99]

Description

Scope: core

Type of change: refactor

Content description:

• remove the io feature from honeycomb-core
• fuse files of the io builder submodule into a single one & move tests
• rename the io dim2 submodule to serialize since it contains only write logic

Additional information

• Breaking change

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.

Project coverage is 77.98%. Comparing base (739dc00) to head (c4567cc).

Files with missing lines	Patch %	Lines
honeycomb-core/src/cmap/builder/tests.rs	96.15%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #255      +/-   ##
==========================================
- Coverage   79.81%   77.98%   -1.84%     
==========================================
  Files          48       47       -1     
  Lines        6747     6676      -71     
==========================================
- Hits         5385     5206     -179     
- Misses       1362     1470     +108     

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