Got the VM up and running! Augmented the documentation.

Got the VM up and running!  Augmented the documentation.

* src/*.[ch]:  Replaced the remaining `SCM_MAKINUM', and changed `SCM_VELTS'
  into `scm_vector_elements ()'.
* src/vm_loader.c (link):  Fixed so that it pushed a variable object on
  the stack.
* src/vm_system.c (variable-ref):  Fixed so that it uses `scm_variable_ref ()'
  and friends.
* module/system/vm/assemble.scm (dump-object!):  Fixed the string case.
* src/vm_engine.h (CONS):  Use `scm_cons' instead of `SCM_NEWCELL'.
* doc/guile-vm.texi:  Added actual instruction definitions, explanations of
  the program invocation mechanism, programs' object tables, etc., in the
  `Instruction Set' chapter.

git-archimport-id: lcourtes@laas.fr--2004-libre/guile-vm--revival--0.6--patch-5