Hashing fixup, equality support, and serialization support

[LilithHafner]

I need these helper functions to facilitate checking if a string parses the same way on different versions of JuliaSyntax. They're generally useful/bugfixes/implement missing functionality that should be expected, so I figure I'll include them in src.

Fixes #449 - a hash-== mismatch bug
Implements #450 - supports equality on SyntaxNode
Implements #451 - supports serialization by implementing read and write on Kind. I used strings instead of UInt16 to represent Kind, see inline comment for more info.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.84%. Comparing base (3dca95e) to head (3820851).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #452   +/-   ##
=======================================
  Coverage   95.83%   95.84%           
=======================================
  Files          13       13           
  Lines        3937     3946    +9     
=======================================
+ Hits         3773     3782    +9     
  Misses        164      164           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.