Add methods that take/return arrays

The recently-added standard library endian conversion functions like [`from_be_bytes`](https://doc.rust-lang.org/std/primitive.u16.html#method.from_be_bytes) and [`to_be_bytes`](https://doc.rust-lang.org/std/primitive.u16.html#method.to_be_bytes) operate on arrays by value rather than slices by reference, which can provide better type safety in some cases.

It would be great if the `ByteOrder` trait were to add similar methods so that code which still needs to be generic over byte order can benefit from the array approach. Concretely, I would make use of those methods in the [`zerocopy::byteorder`](https://docs.rs/crate/zerocopy/0.3.0/source/src/byteorder.rs) module if they were available.

cc @tamird

Start by locating the ByteOrder trait and its existing endian conversion methods, then compare their APIs with Rust's from_be_bytes and to_be_bytes functions. Review the zerocopy::byteorder module to understand the intended generic use case. Done means the trait offers equivalent array-by-value methods for generic byte-order code.

Written by the indexing model from the issue text.