Improve Edwin debugger:

Improve Edwin debugger:

  Make RETURN-TO and RETURN-FROM evaluate the previous expression
  instead of prompting for an expression to evaluate.

  Provide evaluation commands that do and don't evaluate in the dynamic
  state of the continuation.  Commands that evaluate in the dynamic
  state of the continuation will have their output appear wherever the
  program would have sent its output had no error occured, and fluid
  variables will be bound as they would appear in the program.
  Commands that do not evaluate in the dynamic state of the
  continuation will have their output appear in the debugger buffer,
  and fluid variables will be bound as they would upon leaving the
  continuation.

  Rearrange some key bindings.

  Improve handling of restarts.