sheaf authored Jul 20, 2021 and Marge Bot committed Jul 23, 2021

  Three new tests for type-checking plugins:

    - TcPlugin_Nullary, solving a nullary class constraint
    - TcPlugin_Args, providing evidence for a (unary) class constraint
      using arguments supplied to the plugin
    - TcPlugin_TyFam, solving an equality constraint to rewrite
      a type-family application

  More extensive descriptions of the plugins can be found in their
  respective defining modules.