Immutable value objects for dispatch results

[gsteel]

Whilst working on the Mezzio adapter for FastRoute, I noticed it's "impossible" to create dispatch results without writing to soft @readonly properties.

Psalm hates that… so I took the liberty of dropping PHP 8.1, and making the results readonly classes with constructors.

Mainly this is useful in testing…

I've also added 8.5 to CI

Sorry if this patch is doing too much 😬