Looking at [the spec](https://xmpp.org/extensions/xep-0004.html#protocol-field)
it seems valid not to have a `var` attribute set, at least for fields of type
`fixed` that is:
> If the element type is anything other than "fixed" (see below), it MUST
> possess a 'var' attribute that uniquely identifies the field in the context
> of the form (if it is "fixed", it MAY possess a 'var' attribute). The element
> MAY possess a 'label' attribute that defines a human-readable name for the field.
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.
