Add support for string concatenation

tests/integration_test.rs

@@ -349,6 +349,15 @@ fn should_do_strings_cpool_advanced() {
     assert_eq!(29, get_int(last_frame_value))
 }
 
+#[test]
+fn should_do_strings_concat_inline() {
+    let mut vm = VM::new("std");
+    let last_frame_value = vm
+        .run("samples.javacore.strings.concat.trivial.StringConcatInline")
+        .unwrap();
+    assert_eq!(597, get_int(last_frame_value))
+}
+
 #[test]
 fn should_do_trivial_util_arrays() {
     let mut vm = VM::new("std");

tests/test_data/StringConcatInline.java

@@ -0,0 +1,22 @@
+// javac -XDstringConcat=inline  -d . StringConcatInline.java
+
+package samples.javacore.strings.concat.trivial;
+
+public class StringConcatInline {
+    public static void main(String[] args) {
+        int result = concat();
+    }
+
+    private static int concat() {
+        String abc = "abc";
+        String def = "def";
+        String abcdef = abc + def;
+
+        int sum = 0;
+        for (char c : abcdef.toCharArray()) {
+            sum += c;
+        }
+
+        return sum;
+    }
+}

vm/src/execution_engine/engine.rs

@@ -774,6 +774,14 @@ impl Engine {
                     stack_frame.incr_pc();
                     println!("I2B -> {value}B");
                 }
+                I2C => {
+                    let value = stack_frame.pop() as u16;
+
+                    stack_frame.push(value as i32);
+
+                    stack_frame.incr_pc();
+                    println!("I2C -> {value}C");
+                }
                 LCMP => {
                     let b = stack_frame.pop_i64();
                     let a = stack_frame.pop_i64();