#126 BREAKING! allow only functors in var_signal::modify() (any invok…

…able were valid before)

It doesn't make much sense in allowing member pointers and other esoteric invokable here.
Getting rid of `std::invoke()` call here allows not to include `<functional>` and thus improve compilation times (see #126)

include/ureact/signal.hpp

@@ -84,7 +84,7 @@ class var_node final : public signal_node<S>
         // There hasn't been any set(...) input yet, modify.
         if( !m_is_input_added )
         {
-            std::invoke( func, this->m_value );
+            func( this->m_value );
 
             m_is_input_modified = true;
         }
@@ -93,7 +93,7 @@ class var_node final : public signal_node<S>
         // in apply_input
         else
         {
-            std::invoke( func, m_new_value );
+            func( m_new_value );
         }
     }