Extend Bitraversable with firstA and secondA

This is a MR for the CLC proposal to add firstA and secondA to Bitraversable

Please see the proposal for discussion on breakage and utility.

---

Please take a few moments to address the following points:

• Don't think I can add this myself - if your MR may break existing programs (e.g. touches base or causes the compiler to reject programs), please describe the expected breakage and add the user-facing label. This will run ghc/head.hackage> to characterise the effect of your change on Hackage.
• ensure that your commits are either individually buildable or squashed
• ensure that your commit messages describe what they do (referring to tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a [Note][notes] and cross-reference it from the relevant places.
• mentions new features in the release notes for the next release

Once your change is ready please remove the WIP: tag and wait for review. If no one has offerred review in a few days then please leave a comment mentioning @triagers and apply the Blocked on Review label.