[#226] Merge `IsObject` and `IsObjectClass`

Description

Problem: In Indigo.Internal.Object we have both IsObject and IsObjectClass, the latter being a class & instance redefinition of the former (to allow partial application).

However IsObject itself could be written as a class & instance of IsObject' (TypeDecision a) a so there is no need to have both of them.

Solution: Remove IsObjectClass and define IsObject a as a class and instance of IsObject' (TypeDecision a) a.

Related issue(s)

Resolves #226 (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.