Preserve bound vars in match compiler (using lets)

Motivation and Context

The new 13.1-expansion pass substitutes bound pattern variables for fresh ones, which led to a debugger regression: we rely on the original variables making it through to the debugger in order to show the environment to the user.

Description

This restores the user pattern variables by an alternative method to the approach in !2753 (closed), by inserting let nodes instead of doing a substitution. This lets us delete the substitution code (which, side note, is slightly buggy in the presence of shadowing) and ensures that all pattern bound variables will be preserved.

Component

• compiler
• website
• webide
• vscode-plugin
• debugger

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

Debugger: preserve bound variables in pattern matching.

Checklist:

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• 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).