Simplify transaction input types

[Dentosal]

Aspirational PR. Remove complex type-base Coin and Message types and replace them with plain old structs. Currently not backwards-comptabile, needs support for flatten() attribute for canonical serialization.

Checklist

• [ ] Breaking changes are clearly marked as such in the PR description and changelog
• [ ] New behavior is reflected in tests
• [ ] If performance characteristic of an instruction change, update gas costs as well or make a follow-up PR for that
• [ ] The specification matches the implemented behavior (link update PR if changes are needed)

Before requesting review

• [ ] I have reviewed the code myself
• [ ] I have created follow-up issues caused by this PR and linked them here

[Dentosal]

Seems like we're not doing this.