[Starter Pitch] Introducing a `cycled` method to `Sequence`

[xwu (Xiaodi Wu)]

For context, @Lily_Ballard originally introduced this topic in December 2015 in the following thread:

Proposal: CollectionType.cycle property for an infinite sequence

## Introduction Add a new property `cycle` to CollectionType that returns an infinite SequenceType that yields the elements of the collection in a loop. ## Motivation It's sometimes useful to be able to have an infinite sequence. For example, `CollectionOfOne(x).cycle` could be used to have an infinite sequence of a single element (similar to Repeat but without a count). A common use for infinite sequences is zipping with a finite sequence. As far as I'm aware, the stdlib does not currently p…