Merge ext:saltedpasswords into core
The saltedpasswords extension is mandatory since 6.2. Extension core has a dependency to saltedpasswords, a working TYPO3 core can't be achieved without having ext:saltedpasswords next to it. With ext:core being the most basic TYPO3 framework extension, it does not make much sense to have a second extension that is needed by ext:core.
Goal of this epic is to modernise ext:saltedpasswords and getting its main salt classes merged into ext:core as a set of framework classes for salted password handling and storage within the core extension directly.