FEAT: [R/S] preliminary support for [bytes! <size>] definitions in struct

[Oldes]

When working with system and external binding, you very quickly notice, that many structs use byte arrays... for example Windows Socket data:

typedef struct WSAData {
  WORD           wVersion;
  WORD           wHighVersion;
  char           szDescription[WSADESCRIPTION_LEN+1];
  char           szSystemStatus[WSASYS_STATUS_LEN+1];
  unsigned short iMaxSockets;
  unsigned short iMaxUdpDg;
  char FAR       *lpVendorInfo;
} WSADATA, *LPWSADATA;

Or libmpg123's id3 tag:

typedef struct
{
	char tag[3];         /**< Always the string "TAG", the classic intro. */
	char title[30];      /**< Title string.  */
	char artist[30];     /**< Artist string. */
	char album[30];      /**< Album string. */
	char year[4];        /**< Year string. */
	char comment[30];    /**< Comment string. */
	unsigned char genre; /**< Genre index. */
} mpg123_id3v1;

To use these structs from current Red/System is possible, but not easy, so I tried to extend Red/System as you can see in this PR, so for the second example I could use:

mpg123_id3v1!: alias struct! [
	tag     [bytes!  3] ; Always the string "TAG", the classic intro.
	title   [bytes! 30] ; Title string. 
	artist  [bytes! 30] ; Artist string.
	album   [bytes! 30] ; Album string.
	year    [bytes!  4] ; Year string.
	comment [bytes! 30] ; Comment string.
	genre   [byte!]     ; Genre index.
]

[greggirwin]

@PeterWAWood, and thoughts on this PR? I know @dockimbel and @qtxie need to weigh in as well, but it looks like a good idea to me, and includes tests. From the approach taken, it shouldn't affect old code that uses existing syntax.

[greggirwin]

It also doesn't preclude further extension, from a brief evaluation.