Refactor to support strong typing.

[nreynis]

I've added strong typing everywhere it was possible.
I also:

• removed all magic strings for FQCN
• fixed namespaces in tests comment doc (phpStorm is now happy and successfully identify the object types)
• I initially did the same thing for @covers annotations but reverted it because the FQCN is required

Adding strong typing made me spot two bugs that I fixed in the meantime:

• splice wasn't returning this and therefore wasn't fluent (also added doc for this one)
• test for sortKeysWithDefaultSortingAndReverseOption had a bug, you were passing null assuming it would take the default value. This is not how PHP handles that, default values are provided only if you pass nothing at all. Passing an explicit null will give you a null (now a TypeError as the param isn't nullable). As you were passing the second argument you've got no choice: you're forced to pass the first.

[nreynis]

Seems like getFluentTypeErrorForMockedTrait isn't robust enough to pass on lower versions of phpUnit. I'll fix that this weekend.

[nreynis]

Fixed tests and squashed commits

[florianeckerstorfer]

Awesome, thank you so much for the work

[nreynis]

You're welcome.