Fix violation of the Interface Segregation Principle

[antonkomarev]

To avoid violation of Interface Segregation Principles we may extract following interfaces:

• Nullable
• Countable
• Weightable

There are many ways to design package namespace.

1. Keep these interfaces inside of the I\Love namespace
2. Bundle interfaces in helper package like I\Support, I\Entity, etc.
3. Create separate package for each of them:

• I\Nullable
• I\Countable
• I\Weightable

4. Mix two previous points and create bundle package with split sub-packages:

• I\Object\Nullable
• I\Object\Countable
• I\Object\Weightable

[antonkomarev]

Do we need to extract isEqualTo(self $that) & isNotEqualTo(self $that) methods from Reactant, Reacter & ReacterType models to Equalable interface?

Possible package candidate: https://github.com/php-i/comparison