Address code review for credential module

[noandrea]

The list is the following:

• https://github.com/elesto-dao/elesto/pull/94#discussion_r880528258
• https://github.com/elesto-dao/elesto/pull/94#discussion_r880528697
• https://github.com/elesto-dao/elesto/pull/94#discussion_r880530028
• https://github.com/elesto-dao/elesto/pull/94#discussion_r880534185
• https://github.com/elesto-dao/elesto/pull/94#discussion_r880538785

[noandrea]

solved in #146