* lib3.tex (module string): added rindex().

* lib3.tex (module string): added rindex().
* lib1.tex (section{Built-in Functions}): added bagof(), lambda(), map()
  and reduce().  Repharased apply().  Removed or rephrased references to
  exec() (now the exec stmt).
* lib4.tex: posix.exec --> posix.execv
* ref4.tex, ref8.tex, tut.tex: builtin --> __builtin__
* lib3.tex (module string): added atof() and atol(), and ato[fl]_error.