Cosmetic changes from #1737

[JustinDrake]

Pulling out some of the cosmetic changes from #1737:

• Rename the SigningRoot container to SigningData. The signing_root (of type Root) is the hash_tree_root of the SigningData (of type Container).
• Cleanup compute_signing_root
• Remove unnecessary new lines.

(The rationale for this split is because the substantive changes from #1737 may not make it in phase 0. Also this will help focus the discussion in #1737 on the substantive changes.)

[hwwhww]

I changed the target branch to v012x(dev branch for the backward compatible changes)

I know devs hate renamings but this PR makes sense. LGTM 👍

[djrtwo]

I really think we should avoid nitpicking the spec at this point. Any non-security critical changes we introduce add unnecessary overhead for client teams and the potential to introduce minor bugs.

I agree that this looks good and the PR is a very small and digestible. Okay to merge.
But, I reiterate, now is generally not the time for a wave of renamings and code restructurings even if non-substantive