Extend<Pair<&..>> and Pairs::cloned supports for Punctuated

[dhardy]

This is designed to make it easy to extend a Punctuated from an iterator over another Punctuated:

1. Auto-insert extra punctuation if extend-target does not have trailing comma. This is a breaking change due to the extra Default bound, so I may need to remove it.
2. Support extension from reference types.
3. Support cloned over a Pairs iterator.

The last two are redundant with each other, from the POV of the motivation.

[dhardy]

BTW I'm waiting on comment before looking at CI failures. Quite possibly the Default bound needs to be removed?

[dtolnay]

I picked up two of these changes in #1255 and #1256. Thanks!

The rest is too far into diminishing returns I think. For example .map(Pair::cloned) supersedes the need for a new ClonedPairs iterator type.

[dhardy]

Thanks!