bug: fix ni:// translator

[cboettig]

Translation from ni:// needs to support url-encoded base64 strings, with and without percent-encoding.
I'm not entirely sure how to translate those into hexadecimal. e.g. here's an ni:// identifier: ni:///sha256;N6yv_VPrTmQ4Mf6bFfay2XJxqYnr-d2i7QvAMGUsgQA

Here's the ni (base64url-encoded) hash vs the vanilla base64-encoded hash (e.g. as it would look in SRI or SSB):

N6yv_VPrTmQ4Mf6bFfay2XJxqYnr-d2i7QvAMGUsgQA
N6yv/VPrTmQ4Mf6bFfay2XJxqYnr+d2i7QvAMGUsgQA=

The / and + have been encoded as _ and - instead, and the trailing = dropped. I'm not sure how to reverse that as the thing isn't percent-encoded.

(Also I believe according to the spec, ni:///;N6yv%2fVPrTmQ4Mf6bFfay2XJxqYnr%2bd2i7QvAMGUsgQA is also valid (note the percent encoding this time) but even hash-archive.org fails to parse that one.