Create fulfill function for void promises

I find it annoying to have to add a Void value every time I need to fulfill a Void promise. It can also be confusing how to fulfill Void promises for people who are not familiar with how to instantiate a Void value.

Sources/Resolver.swift

@@ -65,6 +65,11 @@ extension Resolver where T == Void {
             fulfill(())
         }
     }
+
+    // Fulfills the promise
+    public func fulfill() {
+        self.fulfill(())
+    }
 }
 #endif