[2.0] Strict types, native parameter & return types

[Majkl578]

Migrating code to PHP 7.1:

• opened 2.0.x-dev
• enabled strict types
• added parameter & return types
• added const visibility
• added brief upgrading notes

No changes in tests needed except one inheritance violation.

There is no develop branch yet, thus targetting master - let me know how to proceed or when to change base branch. 😎

[psfpro]

Hi! What about the slice method? I think slice should return the Collection like filter method.
This will make collection more useful. For example:

$collection->slice(0, 20)->map($func)->toArray();