Switch back to packed_simd dependency

[atouchet]

packed_simd is now again being published under its original name.

[velvia]

Thanks so much @atouchet .
Do you know when they are going to standardize packed_simd under stable?

[velvia]

@atouchet do you mind adding this to Cargo.toml? There is a build failure.

Downloaded libm v0.2.7
22
  Downloaded thiserror v1.0.49
[23](https://github.com/velvia/compressed-vec/actions/runs/6270624969/job/17377301791?pr=5#step:3:24)
error: failed to parse manifest at `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/thiserror-1.0.49/Cargo.toml`
[24](https://github.com/velvia/compressed-vec/actions/runs/6270624969/job/17377301791?pr=5#step:3:25)

[25](https://github.com/velvia/compressed-vec/actions/runs/6270624969/job/17377301791?pr=5#step:3:26)
Caused by:
[26](https://github.com/velvia/compressed-vec/actions/runs/6270624969/job/17377301791?pr=5#step:3:27)
  feature `edition2021` is required
[27](https://github.com/velvia/compressed-vec/actions/runs/6270624969/job/17377301791?pr=5#step:3:28)

[28](https://github.com/velvia/compressed-vec/actions/runs/6270624969/job/17377301791?pr=5#step:3:29)
  consider adding `cargo-features = ["edition2021"]` to the manifest