Fix: Top-level recursive functions for cameligo

Motivation and Context

In case of top level let-rec e.g. let rec x = fun n -> if n = 0 then 0 else x (n - 1)

The recursive call give an error Variable "x" not found. (Because the rec flag was lost in abstractor).

Description

This MR fixes top-level let-rec-fun.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement (non-breaking change that improves performance)
• None (change with no changelog)

Changelog

Fixes top-level recursive functions of the form let rec x = fun y -> ...

Checklist:

• Changes follow the existing coding style (use dune @fmt to check).
• Tests for the changes have been added (for bug fixes / feature).
• Documentation has been updated.
• Changelog description has been added (if appropriate).
• Start titles under ## Changelog section with #### (if appropriate).
• There is no image or uploaded file in changelog
• Examples in changed behaviour have been added to the changelog (for breaking change / feature).