feature: replace const char* with String in socket
demo: add demo of socket::getAddr()
partly support utf8 string
demo: test utf8 for string
feature: add support for string, improve functions, such as lengthWithUTF8 substringWithUTF8
demo: add demo for string::lengthWithUTF8 and string::substringWithUTF8
test: add test for substringWithUTF8 and substring::lengthWithUTF8