[Chore] Split optimizer module

Description

Problem: optimizer module is quite large and hard to work with. Furthermore, since we disable pattern match coverage checker there, the potential for making mistakes is rather high.

Solution: Split into several modules, one filled almost exclusively with rules.

Also add the missing pattern in the Buildable OptimizationStage instance.

Related issue(s)

Resolves None

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.