cxxrtl: make observer methods non-virtual. (breaking change)

This avoids having to devirtualize them later to get performance back,
and simplifies the code a bit.

The change is prompted by the desire to add a similar observer object
to `eval()`, and a repeated consideration of whether the dispatch on
these should be virtual in first place.

backends/cxxrtl/cxxrtl_backend.cc

@@ -2391,7 +2391,7 @@ struct CxxrtlWorker {
 				f << indent << "}\n";
 				f << "\n";
 				f << indent << "bool commit() override {\n";
-				f << indent << indent << "null_observer observer;\n";
+				f << indent << indent << "observer observer;\n";
 				f << indent << indent << "return commit<>(observer);\n";
 				f << indent << "}\n";
 				if (debug_info) {
@@ -2486,7 +2486,7 @@ struct CxxrtlWorker {
 				f << indent << "}\n";
 				f << "\n";
 				f << indent << "bool commit() override {\n";
-				f << indent << indent << "null_observer observer;\n";
+				f << indent << indent << "observer observer;\n";
 				f << indent << indent << "return commit<>(observer);\n";
 				f << indent << "}\n";
 				if (debug_info) {

backends/cxxrtl/runtime/cxxrtl/cxxrtl.h

@@ -865,19 +865,12 @@ struct observer {
 	// Called when the `commit()` method for a wire is about to update the `chunks` chunks at `base` with `chunks` chunks
 	// at `value` that have a different bit pattern. It is guaranteed that `chunks` is equal to the wire chunk count and
 	// `base` points to the first chunk.
-	virtual void on_update(size_t chunks, const chunk_t *base, const chunk_t *value) = 0;
+	void on_update(size_t chunks, const chunk_t *base, const chunk_t *value) {}
 
 	// Called when the `commit()` method for a memory is about to update the `chunks` chunks at `&base[chunks * index]`
 	// with `chunks` chunks at `value` that have a different bit pattern. It is guaranteed that `chunks` is equal to
 	// the memory element chunk count and `base` points to the first chunk of the first element of the memory.
-	virtual void on_update(size_t chunks, const chunk_t *base, const chunk_t *value, size_t index) = 0;
-};
-
-// The `null_observer` class has the same interface as `observer`, but has no invocation overhead, since its methods
-// are final and have no implementation. This allows the observer feature to be zero-cost when not in use.
-struct null_observer final: observer {
-	void on_update(size_t chunks, const chunk_t *base, const chunk_t *value) override {}
-	void on_update(size_t chunks, const chunk_t *base, const chunk_t *value, size_t index) override {}
+	void on_update(size_t chunks, const chunk_t *base, const chunk_t *value, size_t index) {}
 };
 
 template<size_t Bits>
@@ -916,8 +909,7 @@ struct wire {
 
 	// This method intentionally takes a mandatory argument (to make it more difficult to misuse in
 	// black box implementations, leading to missed observer events). It is generic over its argument
-	// to make sure the `on_update` call is devirtualized. This is somewhat awkward but lets us keep
-	// a single implementation for both this method and the one in `memory`.
+	// to allow the `on_update` method to be non-virtual.
 	template<class ObserverT>
 	bool commit(ObserverT &observer) {
 		if (curr != next) {

backends/cxxrtl/runtime/cxxrtl/cxxrtl_replay.h

@@ -556,22 +556,20 @@ public:
 	bool record_incremental(ModuleT &module) {
 		assert(streaming);
 
-		struct : public observer {
+		struct {
 			std::unordered_map<const chunk_t*, spool::ident_t> *ident_lookup;
 			spool::writer *writer;
 
 			CXXRTL_ALWAYS_INLINE
-			void on_update(size_t chunks, const chunk_t *base, const chunk_t *value) override {
+			void on_update(size_t chunks, const chunk_t *base, const chunk_t *value) {
 				writer->write_change(ident_lookup->at(base), chunks, value);
 			}
 
 			CXXRTL_ALWAYS_INLINE
-			void on_update(size_t chunks, const chunk_t *base, const chunk_t *value, size_t index) override {
+			void on_update(size_t chunks, const chunk_t *base, const chunk_t *value, size_t index) {
 				writer->write_change(ident_lookup->at(base), chunks, value, index);
 			}
-		} record_observer;
-		record_observer.ident_lookup = &ident_lookup;
-		record_observer.writer = &writer;
+		} record_observer = { &ident_lookup, &writer };
 
 		writer.write_sample(/*incremental=*/true, pointer++, timestamp);
 		for (auto input_index : inputs) {