Add invoke_lua function to world model

luprex/core/cpp/invocation.cpp

@@ -1,6 +1,15 @@
 
 #include "invocation.hpp"
 
+const std::string &InvocationData::get(const std::string &key) const {
+    static std::string blank_;
+    auto iter = find(key);
+    if (iter == end()) {
+        return blank_;
+    } else {
+        return iter->second;
+    }
+}
 
 void InvocationData::serialize(StreamBuffer *sb) const {
     assert(int(size()) < 65536);