Implement native function pointer check, addr conversion and register, update documents (#185)

Modified WASM runtime API:
- wasm_runtime_module_malloc()
- wasm_runtime_lookup_function()
Introduced runtime API
- wasm_runtime_register_natives()

core/iwasm/aot/aot_reloc.h

@@ -16,6 +16,7 @@ typedef struct {
     REG_SYM(aot_set_exception_with_id),   \
     REG_SYM(aot_get_exception),           \
     REG_SYM(aot_is_wasm_type_equal),      \
+    REG_SYM(aot_invoke_native),           \
     REG_SYM(wasm_runtime_enlarge_memory), \
     REG_SYM(wasm_runtime_set_exception),  \
     REG_SYM(fmin),                        \