fix(clippy): only expose pub functions that need to be public

[Manuthor]

• [x] do not expose functions as pub if not necessary (lint unreachable-pub)
• [x] remove useless async (lint unused_async)
• [x] remove unused self (lint unused_self)
• [x] fix lint or_fun_call:
  • The function will always be called. This is only bad if it allocates or does some non-trivial amount of work.
• [x] fix some nursery clippy lints (lint clippy::nursery)
• [x] fix lint manual_let_else and if_not_else for readability
• [x] some fixes come from Cargo.toml edition in 2024 (all env. variables functions handling for example)