fix: unify infix expressions by isolating unbound type variables

[asterite]

Description

Problem

Resolves #8780

Summary

The explanation of why this works is a bit tricky, but here it is:

• At one point in the compiler (sort_commutative) types are sorted
• The type Type::TypeVariable eventually holds a type variable ID, and that's how these types are sorted
• Depending on the other of type variables in a TypeBindings, these might affect the result of sort_commutative
• In the case where the compiler failed to compile, we ended up with an equation like A + B = 3 - C, and that doesn't unify

Huge thanks to @jfecher for helping me debug this. While debugging, we noticed that even though A + B = 3 - C didn't unify, it could. The "solution" is to say that A = 3 - C - B or that B = 3 - C - A, and that's what this PR does. Now that these unify, it doesn't matter anymore whether we end up with one type or another using sort_commutative, because in both cases we'll be able to unify them.

@jfecher Feel free to improve the above explanation 😅

Additional Context

Documentation

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.