Fixed a stack leak. Now observing actual performance.

Fixed a stack leak.  Now observing actual performance.

* src/*.[ch]:  Replaced `scm_mem2symbol' by `scm_from_locale_symboln' and
  `scm_ulong2num' by `scm_from_ulong'.
* src/vm_system.c (tail-call):  Fixed stack leak (SP lacked decrement by
  one more Scheme object in the tail-recursive case).
* benchmark/measure.scm (measure):  Make sure we are using the compiled
  procedure (i.e. a program object) when measuring.  This yields better
  results than before.  :-)
* doc/guile-vm.texi:  Augmented the instruction set documentation with
  branch instructions, `call' and `tail-call'.

git-archimport-id: lcourtes@laas.fr--2004-libre/guile-vm--revival--0.6--patch-7