[#97] fix `Show` instance and add a `Buildable` one for the `Label` type

Description

In MR !272 (merged) the Show instance for the newly introduced Label type always returns Label.

This modifies the Show instance to return both Label and the Symbol representation and adds a Buildable instance based on the latter only.

Related issue(s)

Fix for #97 (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:
    • README
    • Haddock
    • docs/
  • I updated the changelog if my changes are externally visible.

Stylistic guide (mandatory)

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