chore!: replace dummy ABIs with FunctionSignature type alias

[TomAFrench]

Related issue(s)

Description

Summary of changes

Following #851, we're now exposing methods to compute an "ABI" which doesn't contain half the fields of an actual ABI (namely, the witness maps). We then need some reduced ABI-ish format which just tells us what the arguments/return values are and should look like.

This PR introduces this as a FunctionSignature type alias which just describes the arguments and return values of a function.

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

[jfecher]

👍