Extract `ExpressionKind` from `Expression`

[Centril]

Work towards https://github.com/AleoHQ/leo/issues/1947.

[codecov[bot]]

Codecov Report

Merging #1977 (57d0cce) into testnet3 (ce2d8f8) will decrease coverage by 0.72%. The diff coverage is 83.03%.

:exclamation: Current head 57d0cce differs from pull request most recent head a55fb9f. Consider uploading reports for the commit a55fb9f to get more accurate results

@@             Coverage Diff              @@
##           testnet3    #1977      +/-   ##
============================================
- Coverage     81.88%   81.15%   -0.73%     
============================================
  Files           130      121       -9     
  Lines          5488     5058     -430     
  Branches       5487     5057     -430     
============================================
- Hits           4494     4105     -389     
+ Misses          994      953      -41     

Impacted Files	Coverage Δ
compiler/ast/src/expressions/binary.rs	60.29% <ø> (+0.29%)	:arrow_up:
compiler/ast/src/expressions/call.rs	27.27% <ø> (-6.07%)	:arrow_down:
compiler/ast/src/expressions/circuit_init.rs	37.50% <ø> (-3.68%)	:arrow_down:
compiler/ast/src/expressions/err.rs	0.00% <ø> (ø)
compiler/ast/src/expressions/ternary.rs	57.14% <ø> (-5.36%)	:arrow_down:
compiler/ast/src/expressions/tuple_init.rs	28.57% <ø> (-8.93%)	:arrow_down:
compiler/ast/src/expressions/unary.rs	53.33% <ø> (-1.51%)	:arrow_down:
compiler/ast/src/input/input_ast.rs	65.21% <ø> (ø)
compiler/ast/src/lib.rs	89.70% <ø> (ø)
compiler/ast/src/expressions/mod.rs	62.96% <35.71%> (-7.31%)	:arrow_down:
... and 64 more

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more

[collinc97]

Closing since this PR refactors stale logic