uses ConstZero instead of Default for zero of type T #15333

Job	Run time
check	6m 6s
cargo-test-sbf (token/program)	6m 48s
cargo-test-sbf (instruction-padding/program, token/program-2022, token/program-2022-test)	13m 52s
test (token/cli, token/program, token/program-2022, associated-token-account/program, instruction...	15m 43s
cargo-test-sbf (associated-token-account/program, associated-token-account/program-test)	8m 22s
test (single-pool/cli, single-pool/program)	16m 37s
cargo-test-sbf (token-upgrade/program)	7m 37s
test (token-upgrade/cli, token-upgrade/program)	13m 31s
cargo-test-sbf (feature-proposal/program)	7m 52s
cargo-test-sbf (governance/addin-mock/program, governance/program)	11m 31s
cargo-test-sbf (name-service/program)	8m 6s
cargo-test-sbf (stake-pool/program)	9m 50s
cargo-test-sbf (single-pool/program)	8m 13s
	2h 14m 8s

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