* libguile/vm-i-system.c (long-object-ref, long-toplevel-ref)
(long-toplevel-set): Add new instructions, for accessing the object
table with a 16-bit offset. HTMLprag defines a test program that has
more than 256 constants, necessitating this addition.
* doc/ref/vm.texi: Mention the new instructions.
* module/language/glil/compile-assembly.scm: Emit long refs for object
tables bigger than 256 entries.