Add wasm_runtime_unregister_natives (#1647)

Allow to unregister (or unload) the previously registered native libs,
so that no need to restart the whole engine by using
`wasm_runtime_destroy/wasm_runtime_init`.

core/iwasm/common/wasm_native.h

@@ -64,6 +64,10 @@ wasm_native_register_natives_raw(const char *module_name,
                                  NativeSymbol *native_symbols,
                                  uint32 n_native_symbols);
 
+bool
+wasm_native_unregister_natives(const char *module_name,
+                               NativeSymbol *native_symbols);
+
 bool
 wasm_native_init();