Once again I've missed a `Sequence<T>.peek` ...

# stdlib

[cbruegg]

Once again I've missed a Sequence<T>.peek function today 😄 I think I've found a neat one-liner (I'd bet someone else already discovered it though) to implement it. Would you like me to open a PR or are you not currently interest in such a function?
inline fun <T> Sequence<T>.peek(crossinline f: (T) -> Unit): Sequence<T> = map { f(it); it }