NSString 是为了人类阅读而存在的，必须进行编码，以使得bytes对人类有意义
String Objects
An NSString object encodes a Unicode-compliant text string, represented as a sequence of UTF–16 code units. All lengths, character indexes, and ranges are expressed in terms of 16-bit platform-endian values, with index values starting at 0.
An NSString object can be initialized from or written to a C buffer, an NSData object, or the contents of an NSURL. It can also be encoded and decoded to and from ASCII, UTF–8, UTF–16, UTF–32, or any other string encoding represented by NSStringEncoding.
NSDate是byte数据的直接封装，或者说不关注编码格式；适用于和计算机直接沟通。
NSDate与bytes等价，与编码格式无关，只关注byte流。
NSData and its mutable subclass NSMutableData provide data objects, object-oriented wrappers for byte buffers.