Use 64-bit hashes regardless of platform

[TimSylvester]

Per #1914, use uint64_t for hash keys instead of size_t for more consistent hash results across platforms.

The unit tests check the shader definitions for collisions alongside testing the hash algorithm with random values, but (so far) only in Metal builds where that doesn't require reflection.

Resolves #1917

[github-actions]

Bloaty Results 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0% +3.41Ki  +0.0%    +328    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-1918-compared-to-main.txt

Compared to d387090 (legacy)

    FILE SIZE        VM SIZE    
 --------------  -------------- 
   +18% +21.4Mi  +399% +23.8Mi    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-1918-compared-to-legacy.txt

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a92bc67) 85.74% compared to head (b6b1c55) 85.75%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1918      +/-   ##
==========================================
+ Coverage   85.74%   85.75%   +0.01%     
==========================================
  Files         568      568              
  Lines       28026    28027       +1     
==========================================
+ Hits        24031    24035       +4     
+ Misses       3995     3992       -3     

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