OC 的实现方式：
NSString *a_String = @"A";
NSInteger a_int = [a_String characterAtIndex:0];//65
int b_int = 66;
NSString *b_String = [NSString stringWithFormat:@"%c ",b_int]; // B
Swift 的实现方式：
var a_String:String = "A"
var a_int:UInt16 = (a_String as NSString).character(at: 0)
var b_int:UInt16 = 66;
var b_string:String = String(format: "%c", b_int)
