refactor: kernel circuit refactor

[LeilaWang]

• Change from counter >= split_counter to counter > split_counter
• Use existing utils to get squash flags to build kept arrays
• Remove if-else from general hash utils

[AztecBot]

Flakey Tests

🤖 says: This CI run detected 2 tests that failed, but were tolerated due to a .test_patterns.yml entry.

\033FLAKED\033 (8;;http://ci.aztec-labs.com/6f48fe4010a7847c�6f48fe4010a7847c8;;�):  yarn-project/end-to-end/scripts/run_test.sh simple src/e2e_p2p/upgrade_governance_proposer.test.ts (147s) (code: 1) group:e2e-p2p-epoch-flakes (\033Leila Wang\033: Remove if-else from general hash utils.)
\033FLAKED\033 (8;;http://ci.aztec-labs.com/8ed471011d53d327�8ed471011d53d3278;;�): ./boxes/scripts/run_test.sh vite chromium (604s) (code: 124) (\033Leila Wang\033: Remove if-else from general hash utils.)