Split source code into multiple files without other functional changes

[giordano]

For #75. CC: @oscardssmith.

[codecov]

Codecov Report

Attention: Patch coverage is 98.67841% with 3 lines in your changes missing coverage. Please review.

Project coverage is 98.67%. Comparing base (45d7ae4) to head (3063385).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/FixedSizeArray.jl	98.02%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #76   +/-   ##
=======================================
  Coverage   98.67%   98.67%           
=======================================
  Files           1        2    +1     
  Lines         227      227           
=======================================
  Hits          224      224           
  Misses          3        3           

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