x/ibc ICS24

[fedekunze]

Description

Adds subpackage 24-host/ to the ibc module, that incorporates the changes from the ICS 024 - Host State Machine Requirements spec.

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[cwgoes]

LGTM except that I don't understand the "paths as sets" bit - paths aren't sets.

[cwgoes]

Do we use these identifier validation functions in ValidateBasic on messages elsewhere?

(question from @jackzampolin)

[cwgoes]

Sounds like we don't have a Set type for now, we'll deal with this later.

[alexanderbez]

ACK ☕️