[#228] Add polymorphic constraints to Indigo Expr

Description

Problem: In Lorentz we have an Lorentz.Polymorphic module that contains both of the polymorphic constraints and their instances. However to use them in Indigo Expressions, we need to add additional constraints every time.

Solution: This can be simplified by collecting all the polymorphic constraints in one place in Indigo.Internal.Expr.Types.

Related issue(s)

Resolves #228 (closed)

Depends on #224 (closed)

✅ 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.