pack for Sagittarius

I have committed the pack library. The implementation is influenced Industria's (weinholt struct pack) library. The string format, procedure/macro names and idea for optimisation during macro expansion are taken from it. And I have added indefinite length argument format. But don't look at the code ;-)

The basic use is like this;

(import (binary pack))
;; pack makes bytetevector
;; Fixed length
(pack "4C" 1 2 3 4)  ;; => #vu8(1 2 3 4)
;; (pack "4C" 1 2)   ;; => &syntax

;; Indefinite length
(pack "*C" 1 2 3 4)  ;; => #vu8(1 2 3 4)
(pack "*C" 1 2)      ;; => #vu8(1 2)
;; It can only be allowed for the format position.
;; (pack "*C*S" 1 2) ;; => &syntax

;; pack! sets destructively
(let ((bv (make-bytevector 8)))
  (pack! "*C" bv 0 #xFF #xFF #xFF) ;; => #vu8(255 255 255 0 0 0 0 0)
  ;; the third argument is the offset
  (pack! "*S" bv 4 1 2)            ;; => #vu8(255 255 255 0 1 0 2 0)
  ;; #\x is padding, #\! put next data as big endian
  (pack! "6x!S" bv 0 3)            ;; => #vu8(0 0 0 0 0 0 0 3)
)

Both pack and pack! are macro however it can be passed to apply. (Thanks to R6RS).

I still need to make unpack though...