Removed JsonArray::is<T>(i) and JsonArray::set(i,v)

Removed `JsonObject::is<T>(k)` and `JsonObject::set(k,v)`
Replaced `T JsonArray::get<T>(i)` with `JsonVariant JsonArray::get(i)`
Replaced `T JsonObject::get<T>(k)` with `JsonVariant JsonObject::get(k)`

test/JsonObject/invalid.cpp

@@ -15,7 +15,7 @@ TEST_CASE("JsonObject::invalid()") {
   }
 
   SECTION("AddFails") {
-    obj.set("hello", "world");
+    obj["hello"] = "world";
     REQUIRE(0 == obj.size());
   }