Implemented the From method for all clear variants in Statement

[LucaCappelletti94]

This PR introduces the From trait implementation for all variants of the Statement enum which are evidently mapped 1-1. There are a few variants which are not mappable in such a way, and therefore I refrained from creating one From impl for those. I have also added some .into() in the code where I have seen clear examples of statements conversion which may benefit from using the From trait instead of specifying the Statement variant. There are certainly more such cases, but I thought it was outside of the scope of this PR to replace such things.

This PR closes #2020

[iffyio]

For this I'm thinking it could make more sense to have the impls right next to the struct definitions (similar to how we do for display impls) vs a dedicated file for it?

[LucaCappelletti94]

I created the different module as the main one is ever more massive, so I thought it best not to add even more to it. That being said, if you prefer avoiding specialised sub modules but keep the monolitic one I can copy everything back into it.

[iffyio]

Yeah I think we can move the impls close to the associated struct definitions, indeed it would be nice to split the large files but I imagine splitting based on e.g. statement types or similar dimension grouping related structs in the same file

[LucaCappelletti94]

Done in commit 9ef033f

[iffyio]

LGTM! Thanks @LucaCappelletti94!
cc @alamb