Merge progresses on different branches and clean up

[VietAnh1010]

We have different implementations on some branches. Let's merge them and clean up the repo.

TODO:

• Which are those branches?
• Add tests to detect bugs when we merge the branches
• Fix bugs

[dariusf]

The active branches:

• evaluation: higher-order
• StagedSL: effects
• reset_shift: based on StagedSL
• sleek: new work on using Sleek, based on evaluation

Here's what I think should be the best way to integrate everything.

• Start with evaluation, as tests should already be somewhat working there
• Make a new master branch based on evaluation, which will be the new default branch of the repo
• Merge StagedSL into master, get tests working
• Merge reset_shift into master, get tests working
• sleek is easy to merge and can be done at any time

[dariusf]

I have set up a new master branch based off evaluation, with tests working, and all the changes from the previously-open PRs integrated. It's not yet the default and the CI is also not working yet.

[VietAnh1010]

StagedSL is merged, there is a bug that causes a test to loop.

Let's continue by merging reset_shift into master. We can then fix the bugs and enable CI.