Phase 1 / More test coverage

[can-keklik]

• Extensive tests and language coverage for SELECT expressions
• AST changes
• New module structure

[codecov-commenter]

Codecov Report

Attention: 41 lines in your changes are missing coverage. Please review.

Comparison is base (dd02f63) 69.71% compared to head (1ed41de) 83.35%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
server/src/lang/serializer.rs	87.87%	20 Missing ⚠️
server/src/lang/parser.rs	88.37%	10 Missing ⚠️
server/src/runtime/mod.rs	54.54%	5 Missing ⚠️
server/src/lang/ast/sql.rs	0.00%	3 Missing ⚠️
server/src/runtime/eval.rs	99.70%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #21       +/-   ##
===========================================
+ Coverage   69.71%   83.35%   +13.64%     
===========================================
  Files          23       23               
  Lines        3044     4230     +1186     
===========================================
+ Hits         2122     3526     +1404     
+ Misses        922      704      -218     

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