Replace custom hex/percent-encoding with Ohex and Uri
- ocaml-cbort/fuzz: custom hex_decode → Ohex.decode
- space-ground/web: custom hex_digit + hex_to_bytes → Ohex.decode
- space-ground/lib: custom hex_of_string → Ohex.encode
- ocaml-atp/test: custom hex_of_string → Ohex.encode
- ocaml-crypto/test: custom hex_of_string → Ohex.decode + Ohex.encode
- ocaml-atp/xrpc-server: custom decode_percent → Uri.pct_decode
~100 lines of custom encoding code removed.