Expression and curry_flip

[matiboy]

Following up on @dbrattli 's comment in PR #678 :

another change we want to do is to borrow the curry_flip function from the Expression library to simplify all the operator implementations by removing all the nested functions. Take a look at https://github.com/cognitedata/Expression/blob/main/expression/collections/block.py#L602 Thus you can put source back as the first parameter, and curry-flip will move it as the last parameter curried (returning a function that takes it as the parameter).

1. Expression requires Python 3.9 so we would need to indeed borrow (aka copy) the code.
2. Trying to understand the goal of the change; readability? better typing? re-reading the comment, not sure if this is related to concat_map or more generic?
3. I've written the code below; tests pass, but typing is not happy, I think because merge is typed with Any

@curry_flip(1)
def concat_map(
    source: Observable[_T1], project: Mapper[_T1, Observable[_T2]]
) -> Callable[[Observable[_T1]], Observable[_T2]]:
  return source.pipe(map(project), merge(max_concurrent=1))

[dbrattli]

The problem above is the typing of merge. This PR shows how it can be done: #682. I.e it can be used for all operartors, not just concat_map. Not sure if we should use it in the __init__.py file or just in the operator specific files such as e.g _map.py, or both. We need to try it out. But it will remove a lot of clutter in the operator specific files.