[VM] Maximum object vector size

[Ludovic Courtès]

Hello!

The object vector is limited to 256 elements, which is not so much:

  scheme@(guile-user)> (compile `(define (x)
                                  ,(list 'unquote
                                         (unfold (lambda (i) (> i 255))
                                                 number->string
                                                 1+
                                                 0))))
  ERROR: Value out of range 0 to 255: 256

The error comes from `compile-bytecode.scm' trying to write an
`object-ref' instruction with an argument greater than 255.

This is an issue in practice with functions that contain large literal
lists, as in
http://git.savannah.gnu.org/gitweb/?p=skribilo.git;a=blob;f=src/guile/skribilo/engine/context.scm#l78
 .

A workaround would be to use 24-bit integers for object numbers, which
would make the whole instruction 32-bit long (instead of 16-bit), a
better fit for alignment constraints.

I'll see what can be done here.

Thanks,
Ludo'.