Argon2 is a modern password hashing algorithm. It is significantly more secure
than the previous algorithm (salted SHA512). User logins with Argon2 are
relativly slow and cause significant spikes in CPU and memory (100MB) usage.

Existing passwords are gradually migrated to Argon2 on login.

Previously User used salted SHA512 with OpenLDAP-style prefix syntax and
Signup used crypt. Both models had their own hashing and verification
code. Now both use OpenLDAP-style syntax with support for all traditional
formats including crypt. Salted SHA512 is used for new User and Signup
passwords.

Existing Signup objects are migrated to the new format and remain functional.
User passwords now support gradual migration to another hash algorithm when
it is changed in the future.

This code is planned to be used for database-stored API and OAuth2 client
secrets.