Constant BASE64

Padded base64 encoding

This encoding is a static version of:

# use data_encoding::{Specification, BASE64};
let mut spec = Specification::new();
spec.symbols.push_str("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/");
spec.padding = Some('=');
assert_eq!(BASE64, spec.encoding().unwrap());

It conforms to RFC4648.