Transfer the utility methods responsibility to Keruald OmniTools.

Use PSR-4 namespace Keruald\Core for main features.
The configuration, error global functions and not yet migrated
functions are kept.

Offer a transparent migration path to convert user passwords from legacy MD5
to a modern hash (by default, under PHP 7.2, the default is Argon2i) when
they log in successfully.

Use type hints for signature methods.

Build a small site with a login form and some MD5 password to migrate.

Dump session.