Refactor code

[baentsch]

While reviewing #317 various instances of not really well encapsulated code have been found not only concerned with CompositeSigs, e.g., https://github.com/open-quantum-safe/oqs-provider/pull/317/files#r1516527180. This issue is to suggest and track a refactoring of this code.

At the same time, it would be prudent to review code during such refactoring for memory leakage even though it doesn't seem to be discovered by asan, e.g., #317 (comment)

[baentsch]

Also see thoughts in #526 in this regard (code complexity improvements)

[baentsch]

Composite code added in #317 and #549 warrants a particular review as the exact rationale and design isn't immediately visible from the code (at least for me).