Reduce unnecessary public visibility of some SVM structs and APIs #15854

Job	Run time
cargo-test-sbf (token/program)	7m 7s
check	4m 23s
cargo-test-sbf (instruction-padding/program, token/program-2022, token/program-2022-test)	13m 7s
test (token/cli, token/program, token/program-2022, associated-token-account/program, instruction...	16m 4s
test (single-pool/cli, single-pool/program)	15m 11s
cargo-test-sbf (associated-token-account/program, associated-token-account/program-test)	8m 13s
test (token-upgrade/cli, token-upgrade/program)	14m 11s
cargo-test-sbf (token-upgrade/program)	7m 31s
cargo-test-sbf (feature-proposal/program)	7m 44s
cargo-test-sbf (governance/addin-mock/program, governance/program)	10m 57s
cargo-test-sbf (name-service/program)	7m 9s
cargo-test-sbf (stake-pool/program)	10m 1s
cargo-test-sbf (single-pool/program)	7m 53s
	2h 9m 31s

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