scripted-diff: Rename MainSignalsInstance() class to MainSignalsImpl()

```
-BEGIN VERIFY SCRIPT-
s() { git grep -l "$1" src test doc | xargs sed -i "s/$1/$2/g"; }
s 'MainSignalsInstance' 'MainSignalsImpl'
-END VERIFY SCRIPT-

src/validationinterface.h

@@ -175,10 +175,10 @@ protected:
     friend class ValidationInterfaceTest;
 };
 
-class MainSignalsInstance;
+class MainSignalsImpl;
 class CMainSignals {
 private:
-    std::unique_ptr<MainSignalsInstance> m_internals;
+    std::unique_ptr<MainSignalsImpl> m_internals;
 
     friend void ::RegisterSharedValidationInterface(std::shared_ptr<CValidationInterface>);
     friend void ::UnregisterValidationInterface(CValidationInterface*);