implement group::GEN Leo syntax

[collinc97]

let g: group = group::GEN;

• Implemented core constants to live in AST (compiler/ast/src/types/core_constant.rs)
• Leo parser supports GEN symbol
• Leo AST supports associated constants
• Leo type checker emits group type
• Leo code gen pass emits group::GEN operand compatible with latest AVM operand.
• tests added, expectations updated
• examples/groups/ program updated

[collinc97]

This PR partially addresses #2386

[codecov]

Codecov Report

Merging #2401 (45ffd62) into testnet3 (369db97) will increase coverage by 0.24%.
The diff coverage is 93.75%.

@@             Coverage Diff              @@
##           testnet3    #2401      +/-   ##
============================================
+ Coverage     78.26%   78.50%   +0.24%     
============================================
  Files           152      153       +1     
  Lines          4615     4630      +15     
  Branches       4614     4629      +15     
============================================
+ Hits           3612     3635      +23     
+ Misses         1003      995       -8     

Impacted Files	Coverage Δ
compiler/span/src/symbol.rs	80.76% <ø> (ø)
...iler/passes/src/type_checking/check_expressions.rs	81.78% <80.00%> (-0.04%)	⬇️
compiler/ast/src/types/core_constant.rs	100.00% <100.00%> (ø)
...er/passes/src/code_generation/visit_expressions.rs	82.25% <100.00%> (+0.95%)	⬆️
compiler/passes/src/type_checking/checker.rs	94.52% <100.00%> (+0.11%)	⬆️
...rors/src/errors/type_checker/type_checker_error.rs	79.38% <100.00%> (+0.88%)	⬆️

... and 6 files with indirect coverage changes

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[d0cd]

LGTM!