increase range of relative jumps by aligning blocks to 8-byte boundaries

* libguile/objcodes.c (OBJCODE_COOKIE): Bump again, as our jump offsets
  are now multiplied by 8.

* libguile/vm-i-system.c (BR): Interpret the 16-bit offset as a relative
  jump to the nearest 8-byte-aligned block -- increasing relative jump
  range from +/-32K to +/-240K.
  (mvra): Do the same for the mvra jump.

* libguile/vm.c (really_make_boot_program): Align the mvra.

* module/language/assembly.scm (align-block): New export, for aligning
  blocks.

* module/language/assembly/compile-bytecode.scm (write-bytecode): Emit
  jumps to the nearest 8-byte-aligned block. Effectively our range is 18
  bits in either direction. I would like to do this differently -- have
  long-br and long-br-if, and all the other br instructions go to 8 bits
  only. But the assembler doesn't have an appropriate representation to
  allow me to do this yet, so for now this is what we have.

* module/language/assembly/decompile-bytecode.scm (decode-load-program):
  Decode the 19-bit jumps.

module/language/assembly/compile-bytecode.scm

@@ -77,10 +77,12 @@
     ;; Ew!
     (for-each write-byte (bytevector->u8-list bv)))
   (define (write-break label)
-    (let ((offset (- (assq-ref labels label) (+ (get-addr) 2))))
-      (cond ((>= offset (ash 1 15)) (error "jump too big" offset))
-            ((< offset (- (ash 1 15))) (error "reverse jump too big" offset))
-            (else (write-uint16-be offset)))))
+    (let ((offset (- (assq-ref labels label)
+                     (logand (+ (get-addr) 2) (lognot #x7)))))
+      (cond ((not (= 0 (modulo offset 8))) (error "unaligned jump" offset))
+            ((>= offset (ash 1 18)) (error "jump too far forward" offset))
+            ((< offset (- (ash 1 18))) (error "jump too far backwards" offset))
+            (else (write-uint16-be (ash offset -3))))))
   
   (let ((inst (car asm))
         (args (cdr asm))