Move ExtractIf iterator into its own module

[Kxrma47]

Moves ExtractIf and its Debug, Iterator, and Drop implementations from lib.rs to src/iterators/extract_if.rs. Re-exports smallvec::ExtractIf; crate-visible fields allow the existing SmallVec::extract_if constructor to remain unchanged.

Part of #627. Independent of #628.

Validation: cargo test; cargo test --features 'std rayon serde borsh bytes arbitrary'; cargo clippy --all-targets -- -D warnings; cargo doc --no-deps. Nightly rustfmt is exercised by CI.

[alejandro-vaz]

there will be merge conflicts regarding the imports from lib.rs to the iterators with the other PR

[alejandro-vaz]

I'm withholding both of your PRs until v2.0.0-beta.1 gets released, it's a few hours / ~48 hours max, then they will get merged no worries

meanwhile I'm drafting them

thanks for contributing