A new :stepover command for the debugger

A new :stepover command for the debugger

  Step from statement to statement without leaving the block.
  Tries to do the sensible thing when used on expressions.

      The idea is to:
      1 - Step to the next breakpoint and examine the srcloc
      2 - If it is contained in the same statement block as we were,
          then stop and give control to the user,
        else continue to the next breakpoint
      3 - Repeat from 1. If we reach the end of the statement block,
          i.e. no more ticks in this expression after the current one,
          then step normally.

  Replace statement block with 'declaration block' (of an expression) in the
  pseudo algo. above.
  Let's see how well this idea works in practice...