Correct the compilation warnings

[diseraluca]

The are currently some compilation warning being emitted ( about 15 ).

For example:

warning: associated function is never used: `reserved21`
  --> src/session_id/message.rs:13:5
   |
13 |     reserved21: B1,
   |     ^^^^^^^^^^

warning: associated function is never used: `reserved22`
  --> src/session_id/message.rs:14:5
   |
14 |     reserved22: B1,
   |     ^^^^^^^^^^

warning: associated function is never used: `is_anonymous`
  --> src/session_id/message.rs:16:5
   |
16 |     is_anonymous: bool,
   |     ^^^^^^^^^^^^

Most of those depend on unused fields in the modular-bitfield generated structure and should be resolvable by restricting the code generation of setters and getters.

[diseraluca]

Currently blocked by Robbepop/modular-bitfield#66.

As no answer was given after about a week it was chosen to fork the library to move the issue forward.