Refactor externref related APIs of reference types feature (#971)

Currently when calling wasm_runtime_call_wasm() to invoke wasm function
with externref type argument from runtime embedder, developer needs to
use wasm_externref_obj2ref() to convert externref obj into an internal ref
index firstly, which is not convenient to developer.
To align with GC feature in which all the references passed to
wasm_runtime_call_wasm() can be object pointers directly, we change the
interface of wasm_runtime_call_wasm() to allow to pass object pointer
directly for the externref argument, and refactor the related codes, update
the related samples and the document.

core/iwasm/common/wasm_exec_env.h

@@ -125,10 +125,6 @@ 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