smallvec v2's MSRV is already 1.57.0, which is after the relase of Edition 2021 in 1.56.0.

Apply fix for Cargo.toml dev_dependencies warning given by cargo fix --edition.

Improve code generation by only advancing the iterator in 2 locations

Mirrors the implementation from `Vec::spare_capacity_mut`.

This is useful to write directly into the buffer after `SmallVec::with_capacity`,
removing the overhead of the `SmallVec` model entirely when doing so.