refactor: make the backend implementation more generic (#596)

* merge implementation of backend and theory backend
* make step transitions explicit in backend interface
* add aspif parsing tests

Author: rkaminsk

.gitignore

@@ -10,6 +10,8 @@
 /.vs
 /CMakeSettings.json
 /build
+/_codeql_build_dir
+/_codeql_detected_source_root
 /dist
 /doc/html
 /doc/latex

lib/control/include/clingo/control/solver.hh

@@ -795,7 +795,7 @@ class Solver : public BaseView {
     void accept(Ground::ProfileNode::Visitor const &visit) const { grd_.accept(visit); }
 
   private:
-    class ProgramBackendAdapter;
+    class AspifBackend;
 
     //! States for step transitions.
     //!

lib/control/src/grounder.cc

@@ -405,7 +405,7 @@ auto Grounder::ground(Input::ProgramParamVec const &params, Ground::ScriptCallba
         impl_->project();
         impl_->clear();
     }
-    impl_->out->end_step();
+    impl_->out->end_ground();
     if (p != Input::ProfileFlags::off && !params.empty()) {
         impl_->profile.end_step();
     }