Cleanup parsed AST

[tritao]

Description

This cleans up the parsed AST by doing the following:

Remove unused ReturnStatement.

Refactor Declaration::ImplicitReturnExpression as Expression::ImplicitReturn

During the work on debugging some issues around this refactoring, an issue with the way that implicit/explicit return type-checking is currently realized was found, which is reported in #5518.

Currently this was "worked-around" by special casing the type-checking of implicit returns, just like it is currently done, but we should look into unifying this behaviour in a later PR (I've been looking into it, but all hell breaks loose after an initial try).

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[ironcev]

Great to have this cleanup! And also that it revealed the type checking issue.

Just a small detail, there are three clone() operations in code, of expressions and nodes that are not necessary and are simple to remove.

[tritao]

Good catch, cleaned up those unnecessary clones.