Move crypto.rs into common

- Refactor code and move crypto.rs into the common subdirectory.
- Fixed comments

src/cli/cli_args.rs

@@ -104,6 +104,13 @@ pub struct IntroducerRemoveCommand {
         help = "Certificate to be removed as introducer."
     )]
     pub cert: String,
+
+    #[clap(
+        long = "demote",
+        help = "Remove introducing capabilities, but keep certificate authenticated.",
+        default_value_t = false,
+    )]
+   pub demote: bool,
 }
 
 // Local subcommands