support for index expressions and SUM builtin

[bpowers]

two major pieces here: expanding the lexer + parser to support the full array indexing syntax. Second is supporting the array version of min and max builtins, along with the array-only rank, size, stddev, and sum.

[codecov]

Codecov Report

Attention: 133 lines in your changes are missing coverage. Please review.

Comparison is base (a0ea145) 72.86% compared to head (91a50eb) 77.68%.
Report is 9 commits behind head on main.

❗ Current head 91a50eb differs from pull request most recent head 292a7b2. Consider uploading reports for the commit 292a7b2 to get more accurate results

Files	Patch %	Lines
src/simlin-engine/src/compiler.rs	17.46%	52 Missing ⚠️
src/simlin-engine/src/builtins.rs	48.64%	38 Missing ⚠️
src/simlin-engine/src/units_check.rs	0.00%	23 Missing ⚠️
src/simlin-engine/src/ast.rs	25.00%	12 Missing ⚠️
src/simlin-engine/src/units_infer.rs	33.33%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #70      +/-   ##
==========================================
+ Coverage   72.86%   77.68%   +4.81%     
==========================================
  Files          25       28       +3     
  Lines        6166     8079    +1913     
==========================================
+ Hits         4493     6276    +1783     
- Misses       1673     1803     +130     

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