Version 0.21 replaced base64::decode() with an Engine trait and multiple
structs implementing it for various alphabets, various performance
profiles, etc. It is slightly longer to import but in the end does the
very same thing.
Since Rust 1.32.0 (so basically forever ago) we can use the $(…)?
construct in macros to mean one or zero times this chunk of tokens.
This allows making the last comma optional in lists of things.
