Bump base64 to v0.21

[juntyr]

I've bumped base64 to 0.21 and refactored its usage by defining the used engine, STANDARD, as a private const in one place.

Supersedes #432

• I've included my change in CHANGELOG.md

[juntyr]

?r @torkleyy This is read to me merged if you give the ok

[sdroege]

Not that it matters much, but you might want to look at the data-encoding crate as an alternative. It provides a simpler API.

[juntyr]

data-encoding

Thank you for the recommendation - data-encoding's API definitely looks quite nice and we'll keep the crate in mind.