`lib.rs` iterator modularization

[alejandro-vaz]

lib.rs not only contains the implementation of SmallVec, but all its iterators as well: Drain, ExtractIf, IntoIter, Splice (except the parallel iterator)

the goal is to move each struct with its implementations into a file under src/iterators/ which is already created

expect conflicts regarding visibility

this comes originally from #479 but was cancelled due to its complexity back then. it probably is as complex now but we have to do it anyway

[Kxrma47]

I can take Drain and ExtractIf into src/iterators/, including visibility fixes and tests. Would it be useful to split IntoIter and Splice with another contributor and coordinate one joint PR, or would you prefer separate PRs? I will first check the existing iterator tests and keep behavior unchanged. Happy to follow your preferred split before starting.

[alejandro-vaz]

separate PRs are easier to merge, though either way is fine

I already tried before and what I can remember is that visibility becomes a nightmare, though codebase might have changed since then. so yeah go start now and see what you come across with

[alejandro-vaz]

I've assigned the issue to you

[alejandro-vaz]

brother aren't we missing intoiter and splice as well??

are they more complex or something??

[Kxrma47]

Yes, IntoIter and Splice are still in scope. I started with Drain and ExtractIf as separate PRs to keep each change reviewable, as you suggested. I will prepare separate PRs for IntoIter and Splice too, preserving the public API and existing behavior. Splice uses Drain, so I will account for that dependency and coordinate the shared imports when the pending PRs merge. I have not established that either is inherently too complex; they were simply the next pieces of the issue.