Implement reduce.toString()

[Smoren]

Reduces to a string joining all elements.

• Optional separator to insert between items.
• Optional prefix to prepend to the string.
• Optional suffix to append to the string.

function *toString<T>(data: Iterable<T> | Iterator<T>, separator: string, prefix: string, suffix: string): string

PHP implementation example

Needs to be implemented:

• reduce.toString()
• reduce.toStringAsync()
• Stream.toString()
• AsyncStream.toString()