Since Rust 1.76, and some much older nightly, there have been
improvements to the niche computation, which leads to smaller types
which can encode the same amount of data, variants, and such.
This fixes the tests on this compiler version.
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.
