Testing framework: extend comparison

Motivation and Context

It was reported in #ligo that in the context of the testing framework, variant (sum) types could not be compared for something different of equality/non-equality.

Description

This MR updates the comparison (apply_comparison) in interpreter.ml. Instead of relying in compare_value from Ligo_interpreter stage, we directly re-implement comparison at this point (they have different usages).

We change the output of apply_comparison from value t to int. We rely on raise to signal failure. This allows us to use standard List.sort and similar (otherwise it wouldn't work as recursive calls are guarded with t monad, and binds should be applied).

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

The comparison functions (<, <=, etc.) now can work with variant types.

Example

Given the file test_compare.mligo:

type c = [@layout:comb] | B of int | A of string
type cl = c list

let test_cmp_list = Test.assert ([A "hello" ; A "bye"] > [A "hello" ; B 42])

Before this change

$ ligo run test test_compare.mligo
File "test_compare.mligo", line 4, characters 33-75:
  3 | 
  4 | let test_cmp_list = Test.assert ([A "hello" ; A "bye"] > [A "hello" ; B 42])

"Not comparable"

After this change

$ ligo run test test_compare.mligo 
Everything at the top-level was executed. 
- test_cmp_list exited with value ().

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).
• Examples in changed behaviour have been added to the changelog (for breaking change / feature).