Phase 8.4: Complete AST→MIR Lowering - Basic object operations implemented

Co-authored-by: moe-charm <217100418+moe-charm@users.noreply.github.com>

test_field_operations.nyash

@@ -0,0 +1,19 @@
+box Counter {
+    init { count }
+    
+    pack() {
+        me.count = 0
+    }
+    
+    increment() {
+        me.count = me.count + 1
+        return me.count
+    }
+}
+
+static box Main {
+    main() {
+        local c = new Counter()
+        return c.increment()
+    }
+}