add ,step ,stepi ,next and ,nexti

add ,step ,stepi ,next and ,nexti

* module/system/vm/traps.scm (trap-matching-instructions): New trap,
  just installs a next hook and runs the handler when a predicate
  succeeds.

* module/system/vm/trap-state.scm (add-ephemeral-stepping-trap!): New
  procedure, uses trap-matching-instructions with an appropriate
  predicate to handle step, stepi, next, and nexti repl metacommands.

* module/system/repl/command.scm (step, step-instruction, next)
  (next-instruction): New repl debugger commands.