feat(abi)!: add explicit return type field to ABI.

[TomAFrench]

Related issue(s)

Resolves #678

Description

Summary of changes

Draft pending proper error types.

The ABI now has separate fields for the type and witness assignments of the return value. As a know on effect, the circuit's return value is now no longer included in the InputMap and so is taken as a separate argument to Abi.encode and returned separately by Abi.decode.

Some care needs to be taken now when checking to see if an ABI contains a public input as this could either be a public param or a return value, whereas previously we could just search abi.parameters for a public parameter. I've added a getter to perform this check for us.

Dependency additions / changes

Test additions / changes

Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt with default settings.
• I have linked this PR to the issue(s) that it resolves.
• I have reviewed the changes on GitHub, line by line.
• I have ensured all changes are covered in the description.
• This PR requires documentation updates when merged.

Additional context