Refactor: change set/get_decryption_key return type to S2N_RESULT in s2n_cipher struct

[jouho]

Resolved issues:

Part of #3983

Description of changes:

• Change return type of set_decryption_key and get_decryption_key from int to S2N_RESULT
  

  s2n-tls/crypto/s2n_cipher.h

  Lines 87 to 88 in d538d0a

  	int (*set_decryption_key)(struct s2n_session_key *key, struct s2n_blob *in);
  	int (*set_encryption_key)(struct s2n_session_key *key, struct s2n_blob *in);

• Majority of changes are to adapt to correct return types, like converting S2N_SUCCESS to S2N_OK, POSIX_GUARD to POSIX_GUARD_RESULT, and etc.

Call-outs:

Testing:

Confirmed all unit tests pass. Will address any failures in CI.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[maddeleine]

Thanks for cleaning this stuff up :)