fix: `missing_debug_implementations`

[JonathanWoollett-Light]

Sets missing_debug_implementations to warn and adds missing std::fmt::Debug implementations.

When tracing/logging/debugging a program it is common to debug print inputs and outputs of function, to do this generally dependencies need to offer std::fmt::Debug implementations.

I won't repeat what is written under missing_debug_implementations which gives some more explanation.

[JonathanWoollett-Light]

Blocked on https://github.com/RustCrypto/traits/pull/1315

[JonathanWoollett-Light]

Blocked on https://github.com/RustCrypto/block-modes/pull/46

[newpavlov]

This PR got outdated, so I will close it in favor of tracking issue #599.