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.

doc/export_native_api.md

@@ -85,10 +85,11 @@ The function signature field in **NativeSymbol** structure is a string for descr
 
 Each letter in the "()" represents a parameter type, and the one following after ")" represents the return value type. The meaning of each letter:
 
-- '**i**': i32 or externref (for externref, developer should use `wasm_externref_obj2ref()` to map host object to externref index firstly)
+- '**i**': i32
 - '**I**': i64 
 - '**f**': f32
 - '**F**': f64
+- '**r**': externref (has to be the value of a `uintptr_t` variable)
 - '**\***': the parameter is a buffer address in WASM application
 - '**~**': the parameter is the byte length of WASM buffer as referred by preceding argument "\*". It must follow after '*', otherwise, registration will fail
 - '**$**': the parameter is a string in WASM application