Mark a lot of functions const (where possible)

libsolidity/interface/CompilerStack.h

@@ -88,7 +88,7 @@ public:
 		m_errorReporter(m_errorList) {}
 
 	/// @returns the list of errors that occured during parsing and type checking.
-	ErrorList const& errors() { return m_errorReporter.errors(); }
+	ErrorList const& errors() const { return m_errorReporter.errors(); }
 
 	/// @returns the current state.
 	State state() const { return m_stackState; }