Remove manual map implementation in MaybeMath

[TimJentzsch]

Objective

The implementations of MaybeMath used #[allow(clippy::manual_map)].
This PR removes this lint suppression and applies cargo clippy --fix to use map instead of the match statement.

Context

The original implementation justified the lint suppression as follows:

The logic here is subtle and critical;
explicit match statements make it more clear what's going on

Feedback wanted

Do the explicit map statements really make it more clear what's going on?
Personally, I think the map statements are also clear.
If we're unsure if something works we should add unit tests instead.
I think it's good to suppress as little lints as possible.

[alice-i-cecile]

I'm fine with this as long as we solve #158 at the same time :)

[Weibye]

I'm fine with the changes as long as we add both some docs and tests that together explain exactly what they do. I've just started learning about .map these last days so my reading comprehension is not yet at a level where I look at the implementation and intuitively know what's up.

[TimJentzsch]

I'm fine with the changes as long as we add both some docs and tests that together explain exactly what they do. I've just started learning about .map these last days so my reading comprehension is not yet at a level where I look at the implementation and intuitively know what's up.

Hmm it's definitely a fair point that .map is harder to understand for users new to Rust / functional programming, however I'm not sure if adding documentation from std functions to our code base is a good idea

[TimJentzsch]

Looks like the tests are passing, now we still need to figure out if this is easier or harder to read/understand

[alice-i-cecile]

Given that this does the obvious thing + we have unit tests and docs now, I'm going to merge this.

I don't love keeping clippy allows around either.