* Do pointer alignment checks in a way where intermediate steps aren't defined to be Undefined Behavior.
* Update the `packed_simd` dependency to `packed_simd_2`.
* Update the `cfg-if` dependency to 1.0.
* Address warnings that have been introduced by newer Rust versions along the way.
* Update to edition 2018, since even prior to 1.0 `cfg-if` updated to edition 2018 without a semver break.