Switch base64 decoding to data-encoding crate

[sdroege]

It was already previously used elsewhere in this crate and depending on two base64 implementations doesn't seem ideal.

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Also fixes #134

[chifflier]

Oh, I missed this when merging #138

This looks interesting, but we'd need to ensure that the features provided are really similar. Do you think this is the case?
If yes, could you rebase and resubmit? Thanks!

[sdroege]

It's the same base64 variant, if that's your question :)

https://docs.rs/data-encoding/latest/data_encoding/ has a comparison between data-encoding, base64 and GNU base64 for some other properties.

If this sounds good to you I'll rebase the PR.

[chifflier]

Yes, in the past I encountered some weird base 64 variants (some remove the trailing ==, others are said web-safe, etc) and I did not check the RFC to find which is used in certificates. It does not seem to be an exotic one.
I'm fine switching crates, so please go ahead

[sdroege]

Ok, I've rebased this now. Let me know if this look ok to you :)

[chifflier]

Merged, thanks!