Release v5.0.0

In addition to new functionality, this release contains backwards-incompatible changes.
Read about BC here: https://github.com/fp4php/functional/blob/master/UPGRADING-v5.md

New collection ops

• groupMap: groupBy + map combinator
• groupMapReduce: groupBy + map + fold combinator
• reindex: Index each elements by callback and returns HashMap
• firstMap: Like first but use Option data type instead bool
• lastMap: Like last but use Option data type instead bool
• traverseOption, traverseEither: See example
• sorted: $callback now is optional. Will be use default PHP comparison mechanism without $callback.
• sortedBy: allow specify value for sorting.
• sortedDesc: allow specify value for ascending sorting.
• sortedDescBy: allow specify value for desceding sorting.
• flatten: Turns iterable<iterable<T>> to iterable<T>
• zipWithKeys: Turns Seq<T> to Seq<array{int, T}>
• init: Returns all elements except last
• partition: Separates collection by predicate
• partitionMap: Separates collection by predicate (but using Either)
• toString: Returns string representation of datatype.
• toStream: Turns collection to Stream
• toMergedArray: Like array_merge but as method
• toNonEmptyMergedArray: Non-empty version of toMergedArray
• *N combinators: (mapN, flatMapN and others...)

Psalm plugin improvement

• first: type narrowing from predicate.
• last: type narrowing from predicate.
• filter: type narrowing support for first-class callable
• filterNotNull: shape inference
• partitionT: exhaustive type inference
• Either::filterOrElse: type narrowing from predicate.

Docs

• Doc about combinators
• More examples in Monad.md
• Doc about Psalm plugin