Secret/public key wrapper guidelines

## Description

Secret/public key wrapper guidelines are needed: naming convention, interoperability traits, security concerns.

## Motivation

Wrappers for asymmetric and secret keys are common (ed25519, x25519, slip10). Often they use different names and argument/result types to do the same task and implement (or not) different traits, this is inconvenient. Also, secret key bytes can be leaked via local variables which is discouraged.

## Requirements

This issue should introduce a guideline (maybe, a wiki page) and check the current implementations for conformance. A draft can be found here: [wrappers-guidelines](https://github.com/iotaledger/crypto.rs/discussions/145).

## Are you planning to do it yourself in a pull request?

Yes.