Add peek extension method for `Iterable` and `Iterator`

[Boereck]

Analogous to the JDK method java.lang.Stream#peek(java.util.function.Consumer) introduce a peek extension method for Iterable and Iterator. This method just consumes an element and produces an Iterable/Iterator providing the same elements as the source. This method is mainly interesting for debugging purposes.

• static def <T> Iterable<T> peek(Iterable<T> iterable, (T)=>void action)
• static def <T> Iterator<T> peek(Iterator<T> iterator, (T)=>void action)

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