fix: add dark::Visitor to tools & make default ctor implicit for reflection

include/register.h

@@ -28,7 +28,7 @@ struct Register {
   public:
     static constexpr std::size_t _Bit_Len = _Len;
 
-    explicit Register() : _M_old(), _M_new(), _M_assigned() {}
+    Register() : _M_old(), _M_new(), _M_assigned() {}
 
     Register(Register &&) = delete;
     Register(const Register &) = delete;

include/tools

@@ -15,3 +15,4 @@ using dark::Register;
 
 using dark::SyncTags;
 using dark::sync_member;
+using dark::Visitor;

include/wire.h

@@ -75,7 +75,7 @@ struct Wire {
   public:
     static constexpr std::size_t _Bit_Len = _Len;
 
-    explicit Wire() :
+    Wire() :
         _M_func(new details::EmptyWire),
         _M_cache(), _M_holds(), _M_assigned() {}