refactor(farbfeld): use the built-in methods to convert the bytes between different representations

[0x61nas]

I noticed that you are using the byteorder crate to convert the bytes between different representations, Which a thing can be done with the built-in methods like to_le_bytes and from_le_bytes since Rust 1.32.

So I updated the Farbfeld encoder and decoder to use them. However, we still have to depend on byteolder
crate since other encoders/decoders like webp, tga, pnm and ico using its methods, but I can work on that if this PR gets accepted.

I license past and future contributions under the dual MIT/Apache-2.0 license,
allowing licensees to choose either at their option.