Refine annotate and Targetable (backport #4698)

[chiselbot]

Some minor improvements to the user API for annotating with Targetable

• Support annotating Seq[Targetable] and provide Targetable for AnyTargetable.
• Add AnyTargetable.apply for simpler manual conversions.
• Show tests annotating Seq[Targetable] and manual use of AnyTargetable.

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• No Release Notes

Changes to unreleased API

Desired Merge Strategy

• Squash

Release Notes

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash) and clean up the commit message.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.

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This is an automatic backport of pull request #4698 done by [Mergify](https://mergify.com).