modularize `comparisons.rs` and `conversions.rs`

[alejandro-vaz]

split from the src/lib.rs the following two files:

• src/comparisons.rs => contains Eq and Ord for SmallVec, total and partial
• src/conversions.rs => contains all the From and TryFrom implementations related to SmallVec

[Dazzanova]

Hi @alejandro-vaz, I'd like to work on this issue, Im planning to move the Eq/Ord/PartialOrd impls into src/comparisons.rs and the From/TryFrom impls into src/conversions.rs, without any logic changes. Let me know if I should go ahead and raise a PR.

[alejandro-vaz]

yeah go ahead