factors decoder into separate component

gen_input/templates/CORENAME.h.gtl

@@ -69,6 +69,7 @@ def getCString(def val){
 #include <array>
 #include <iss/arch/traits.h>
 #include <iss/arch_if.h>
+#include <iss/arch/instruction_decoder.h>
 #include <iss/vm_if.h>
 
 namespace iss {
@@ -120,6 +121,8 @@ template <> struct traits<${coreDef.name.toLowerCase()}> {
         ${instr.instruction.name} = ${index},<%}%>
         MAX_OPCODE
     };
+
+    static std::unique_ptr<instruction_decoder<${coreDef.name.toLowerCase()}>> get_decoder();
 };
 
 struct ${coreDef.name.toLowerCase()}: public arch_if {