cxxrtl: WIP: adjust comb display cells to only fire on change

Naming and use of statics to be possibly revised.

tests/fmt/always_comb_tb.cc

@@ -0,0 +1,14 @@
+#include <iostream>
+#include "yosys-always_comb.cc"
+
+int main()
+{
+	cxxrtl_design::p_top uut;
+
+	for (int i = 0; i < 20; ++i) {
+		uut.p_clk.set(!uut.p_clk);
+		uut.step();
+	}
+
+	return 0;
+}