ICS 4: Messages split

[adizere]

Part of cosmos/ibc-rs#105

This PR only handles effectively splitting the messages into separate modules.
PR #391 handles the addition of domain types and new tests.

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For contributor use:

• Updated the Unreleased section of CHANGELOG.md with the issue. not needed yet since no issue is closed.
• If applicable: Unit tests written, added test to CI.
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Updated relevant documentation (docs/) and code comments.
• Re-reviewed Files changed in the Github PR explorer.

[ancazamfir]

Looks good! Haven't checked all line by line though :) I assume nothing else has changed.