Introduce method `each` in CollectionTrait

• Introduce method each in CollectionTrait to iterate through each item of the collection.

• Update documentation.

Add AbstractItem

• Add AbstractItem
• Update documentation

Use different approach to abstract collections

• Use different approach to abstract collections (regular and filtered)
• Update README.md

Fix CollectionTrait::fromIterable

• Fix CollectionTrait::fromIterable to allow use by AbstractCollection and AbstractFilterableCollection

Abstract base collections (both regular and filterable)

• Add abstract base collections (both regular and filterable)
• Update README.md

Collection Traits

• Add CollectionTrait and FilterableCollectionTrait

• Define Convertible interfaces: ToArray, ToInt and ToString

• Define Comparable interfaces: Equals

• Define CollectionFilter interface

• Define FilterItem interface

• Define FilterOperator interface

• Add FilterOperatorAnd and FilterOperatorOr

• Add BaseFilter and CompositeFilter