CameLIGO: support for `let mut`, assignment and iteration

Motivation and Context

Description

We update and add support for let mut, as in !2254 (merged) (which was later reverted).

A difference with previous MR is that we use upto instead of to in order to prevent breaking all contracts.

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

Add mutable variables, the assignment operator, for loops, and while loops to CameLIGO.

let do_something g n = 
  let mut x = 1 in
  for j = 1 upto n do
    while x < 10 do
      x := g i j x;
    done
  end;
  x

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).