refactor: hide sec module (#22)

I don't think there's any real need to make this module public, since
it is only a set of utility functions used internally.

BREAKING CHANGE: The sec module was made private

src/lib.rs

@@ -18,7 +18,7 @@ pub mod error;
 pub mod hmacmode;
 mod manager;
 pub mod otpmode;
-pub mod sec;
+mod sec;
 
 use aes::cipher::generic_array::GenericArray;