Choice of PRF / PRP for pseudonym "hardening"

[jschneider-bensch]

          Let's open an issue for the choice of PRF.

Originally posted by @franziskuskiefer in #11 (comment)

ScrambleDB requires a PRF / PRP to convert "raw pseudonyms", i.e. group elements that are output by the coPRF into "hardened pseudonyms". We should make a well-motivated choice.

[jschneider-bensch]

We settled on ChaCha20. Give documentation for this choice.