R4R: x/auth module spec / code reconciliation

[cwgoes]

Closes #2963

This PR

• Adds initial version of x/auth module spec (will likely need to be unified with [R4R]: Vesting Account(s) Implementation #2694)
• Changes account store prefix key to 0x01 instead of "account:" for efficiency and consistency.

I think x/auth could use some more cleanup on top of this, but I'd like to get an initial spec merged as one of our auditors is waiting on it.

Standard checklist

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added entries in PENDING.md with issue #
• rereviewed Files changed in the github PR explorer

---

For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #2964 into develop will increase coverage by 0.02%.
The diff coverage is 50%.

@@             Coverage Diff             @@
##           develop    #2964      +/-   ##
===========================================
+ Coverage    55.53%   55.55%   +0.02%     
===========================================
  Files          120      120              
  Lines         8490     8490              
===========================================
+ Hits          4715     4717       +2     
+ Misses        3453     3451       -2     
  Partials       322      322

[alexanderbez]

One minor comment, but otherwise, LGTM 👌

[jackzampolin]

Good to see some spec getting written! Great work @cwgoes!