entry_to_string function implemented

[KlowdfurrRad]

Implemented entry_to_string function.
Wrote 3-4 simple tests for it.
Wrote documentation for entry_to_string in /include/stumpless/entry.h

Let me know what changes I should make.

#448

[codecov]

Codecov Report

Attention: Patch coverage is 85.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 90.51%. Comparing base (1d2df58) to head (34783a7).

Files with missing lines	Patch %	Lines
src/entry.c	85.00%	7 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           latest     #465      +/-   ##
==========================================
- Coverage   90.59%   90.51%   -0.08%     
==========================================
  Files          47       47              
  Lines        4379     4439      +60     
  Branches      587      594       +7     
==========================================
+ Hits         3967     4018      +51     
- Misses        279      286       +7     
- Partials      133      135       +2     

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