refactor!: have knuu as an object

[mojtaba-esk]

This PR proposes a structural change to the knuu pkg to have it as an object.
This helps to have more idiomatic code and enable us to add unit tests easier. Moreover, since every test can have their own knuu object and handle everything in isolation.

Note 1: in order to keep it backward compatible with the current tests, some wrapper functions with a global var is added, and they are in a number of files with _old.go postfix so we can delete the files when the users are ready to use this one.

Note 2: There was an attempt not to change everything, i.e. the refactor tries to focus on introducing an object structure for knuu. So, we need a number of refactor rounds on the knuu pkg and in general on the repo.

Closes #328

[MSevey]

Since this is a relatively nuanced why of keeping backwards compatibility I would recommend you add some comments to the top of the _old files and their corresponding files that captures how the files are setup and should be used while we are in this period of maintaining backwards compatibility.

Otherwise I think it puts a lot of pressure and burden on the team to keep the context of this refactor in their minds while working on fixes and features.

[smuu]

Is it possible to add a deprecation annotation to the old functions so that we see in the code when we use those functions?

[mojtaba-esk]

Is it possible to add a deprecation annotation to the old functions so that we see in the code when we use those functions?

good idea, done.

[MSevey]

lgtm. I think all open discussions can be turned into follow up issues 👍🏻

[tty47]

you 🪨 🙌