Implements the SipHash PRF

[shikokuchuo]

Adds siphash13().

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.59036% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 98.92%. Comparing base (8c3f751) to head (240cfa6).

Files	Patch %	Lines
src/secret3.c	97.51%	4 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
- Coverage   99.66%   98.92%   -0.75%     
==========================================
  Files           4        5       +1     
  Lines         301      463     +162     
==========================================
+ Hits          300      458     +158     
- Misses          1        5       +4     

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