Remove use of arrayref
Now that const generics and TryInto are in, we have better ways to express converting slices into arrays.
Closes #872 (closed).
Please don't merge until after the 1.1.5 release.
Note: right now, Rustc doesn't optimize these as well as it should with the options set in our release
profile. See discussion at #872 (closed) and at https://github.com/rust-lang/rust/issues/111959 , and workaround patch at https://github.com/rust-lang/rust/pull/111966 . Do we think this is okay?