Psalm5 support

[veewee]

Psalm v5 is out!

This PR contains:

• fixes for psalm v5 (it now has immutable non-cloneable union types)

PSL itself cannot use psalm5 unless roave/infection-static-analysis-plugin supports psalm v5.

• [x] https://github.com/Roave/infection-static-analysis-plugin/pull/461

There are 30-ish new psalm warnings in psalm v5 atm which need to be addressed in the psl package.

[Ocramius]

The infection plugin is updated, BTW 👍

[veewee]

Cool, Thanks! The psalm 5 issues in here and PSL look good to go to me as well. Let's see if @azjezz has something to add.

[azjezz]

lgtm ✅