Representation of rules changed from command list to script

* rules are now represented as a big string with antiquotations inside

* we do not try to cut it into individual commands, which gives more
  freedom when writing scripts (why not write scripts in any language
  you like?)

* the syntax in <:script< >> quotations changed a little, notably the
  antiquotations are now introduced by # instead of %

* the adaptation for the combinator module (Bistro_workflow.Script)
  gives nice expressions like begin_ cmd "echo" arg string "yo" end_

* examples were updated and tested