WIP: Bonded proof-of-stake design specification

[cwgoes]

Closes #1678
Closes #1256
Closes #1670

This will be a (rough) design specification for bonded-proof-of-stake focused on the economic security model (what the costs of breaking agreement or breaking liveness are).

Regular checklist:

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• 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

❗ No coverage uploaded for pull request base (develop@386116f). Click here to learn what that means.
The diff coverage is n/a.

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  Coverage           ?   60.44%           
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  Files              ?      196           
  Lines              ?    14488           
  Branches           ?        0           
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  Hits               ?     8757           
  Misses             ?     5150           
  Partials           ?      581

[cwgoes]

Ported to cosmos/ibc#91