chore: remove duplicate span from FunctionReturnType

[alexvitkov]

Description

Problem*

We have a struct called FunctionReturnType, that has a UnresolvedType and a Span. Recently we added a Span to UnresolvedType itself, so the Span is no longer needed

Summary*

Documentation

• This PR requires documentation updates when merged.

  • I will submit a noir-lang/docs PR.
  • I will request for and support Dev Rel's help in documenting this PR.

Additional Context

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[ghost]

The default variant of the enum was needed in order not to display help in such cases; I guess it would be confusing, not sure.

error: expected type (), found type Field
  ┌─ main.nr:1:14
  │
1 │ fn test() -> () {
  │              -- expected () because of return type
2 │     42
  │     -- Field returned here
  │
  = help: try adding a return type: `-> Field`

[alexvitkov]

Ah ok, makes sense -- I thought it was just for the Span. In that case I'll bring back the struct and just remove span, since we're storing it twice in the struct.

[jfecher]

👍