Import reference-types feature (#612)

Implement spec reference-types proposal for interpreter, AOT and JIT, update documents and add sample. And upgrade AOT_CURRENT_VERSION to 3 as AOT file format and AOT module instance layout are changed.

Signed-off-by: Wenyong Huang <wenyong.huang@intel.com>

core/iwasm/common/wasm_exec_env.h

@@ -116,6 +116,10 @@ typedef struct WASMExecEnv {
     WASMJmpBuf *jmpbuf_stack_top;
 #endif
 
+#if WASM_ENABLE_REF_TYPES != 0
+    uint16 nested_calling_depth;
+#endif
+
 #if WASM_ENABLE_MEMORY_PROFILING != 0
     uint32 max_wasm_stack_used;
 #endif