Implement base64 URI encoding/decoding for Pijul URIs

No user-facing changes but should enable implementing non-file URIs and more complex queries such as "state in a particular channel" or "state with a given set of hashes".

Created by  finchie  yesterday at 21:50
NEAR63OCPUGLXZ46YRBBY5DI5LCVH27XNGGZD7LCMUESHON54JFQC
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