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()

samples/gui/wasm-runtime-wgl/linux-build/CMakeLists.txt

@@ -42,7 +42,6 @@ include_directories(
 set (SOURCES
      ${PROJECT_SRC_DIR}/main.c
      ${PROJECT_SRC_DIR}/iwasm_main.c
-     ${PROJECT_SRC_DIR}/../../ext_lib_export.c
      ${LV_DRIVERS_SOURCES}
      )