[FunC] Forbid auto-creating undefined symbols

crypto/func/auto-tests/tests/invalid-undef-1.fc

@@ -0,0 +1,12 @@
+int main(int x) {
+    return 1 + demo();
+}
+
+int demo() {
+    return 2;
+}
+
+{-
+@compilation_should_fail
+@stderr undefined symbol `demo`
+-}

crypto/func/auto-tests/tests/invalid-undef-2.fc

@@ -0,0 +1,10 @@
+int main(int x) {
+    return 1 + demo;
+}
+
+global int demo;
+
+{-
+@compilation_should_fail
+@stderr undefined symbol `demo`
+-}

crypto/func/auto-tests/tests/invalid-undef-3.fc

@@ -0,0 +1,10 @@
+int demo(int x);
+
+int main(int x) {
+    return 1 + demo(x);
+}
+
+{-
+@compilation_should_fail
+@stderr function `demo` is just declared, not implemented
+-}