Make chunked(on:) include subject value in Element type

[natecook1000]

This PR changes the result of calling chunked(on:) to [(Subject, SubSequence)], and adds a new collection type as the result of lazy.chunked(on:) with a matching element type. Including the projected Subject is often useful when chunking on a projected value like this.

The new versions of chunked(on:) looks like this:

extension Collection {
  public func chunked<Subject: Equatable>(
    on projection: (Element) throws -> Subject
  ) rethrows -> [(Subject, SubSequence)]
}

extension LazyCollectionProtocol {
  public func chunked<Subject: Equatable>(
    on projection: (Element) throws -> Subject
  ) rethrows -> ChunkedOn<Self, Subject>
}

I've also changed the name Chunked to be ChunkedBy, since this PR introduces ChunkedOn.

Checklist

• [x] I've added at least one test that validates that my change is working, if appropriate
• [x] I've followed the code style of the rest of the project
• [x] I've read the Contribution Guidelines
• [x] I've updated the documentation if necessary

[natecook1000]

@swift-ci Please test