Start migrating to SAML2 getters/setters

This reverts commit 9e98109b.

This prevents a lot of -dev dependencies in composer.lock which are
not used otherwise and generate 'scary' warnings like "Package x is
abandoned".

Closes #718

This reverts commit b9f92f33.

This reverts commit 82a46a7c.

This removes the dependency on mcrypt for good, closes #660 and resolves #615.

I believe #592 broke unit tests by adding a dev-dependency to
`composer.json` but forgot to update `composer.lock`. Therefore,
`composer install` does not install all the necessary dependencies for
tests.