Split-up lib.rs

[regexident]

This PR splits up the lib.rs file, introducing separate files for:

• Relation<T> -> relation.rs
• Iteration -> iteration.rs
• Variable<T> -> variable.rs

Splitting up the single 555loc file required me to make a select few previously private bindings pub(crate), instead. However I think overall this greatly improves the accessibility of the project and is worth it.

---

This is part #1 of a stacked pull-request:

└── Split-up\ lib.rs 👈🏻
    └── Cleanup\ generics (#33)
        └── Support arbitrary tuples (#34)

[ecstatic-morse]

Hi @regexident!

I'm in full agreement with this PR, however I think the only active person with r+ rights in this repo is @nikomatsakis, and they are quite busy. I'll try to convince them to give either @lqd or me (preferably both) permission to approve PRs so we can get things moving.

[ecstatic-morse]

I have some stuff in the pipeline that will conflict with this, so I went ahead and added the nit on top before merging (I would have rebased but it's non-trivial to preserve authorship info this way). Thanks again @regexident.

[regexident]

Thanks @ecstatic-morse!